Strategy
So I'm gonna give a bit of a summary of the strategy for achieving this feat here. Quite an interesting challenge for EU3 veterans only, I'd say. But it works.
Note that the steps labeled '(Orthodoxy)' only apply if you want to convert your entire empire from Catholic to Orthodox once you've formed it, which I'm doing as I think it's an interesting idea.
For starters, play as Bohemia. You start out as Emperor, and you need to be Emperor to push Orthodox religion on the HRE electors - they'll never vote for a heretic.
Although, it might work - possibly even better actually - playing as Burgundy, because it takes about 20 years anyway to butter up enough various HRE states to accept your first Imperial Reform, and Bohemia's leader tends to die within about that period of time. You could get elected as Emperor, and delay your change to Orthodox until then. Still, for this attempt, I played as Bohemia.
Keep restarting until you start out with the "Relations with the Habsburgs" mission; this lets you quickly drop 2 infamy later on if you need to (infamy is poisonous to various aspects of forming the HRE, and staying at ZERO infamy is highly desirable).
There shouldn't be a need to ally with any large nations, for defensive purposes. The only alliances formed should be with small nations you diplo-vassalize (and possibly Austria, who can be quite useful in wars with states like Hungary, because they're right next door to you).
Normally, I'd say that allying with and diplo-vassalizing lots of small states within the HRE is bad for forming the HRE, because it tends to ossify things; you want wars to happen so states can take territory resulting in Imperial Liberation CBs that you can then use to get Imperial Authority. But because this strategy lets you get all your Imperial Authority through enforcing religious unity, this is no problem. Ally with small states, form royal marriages, and diplo-vassalize away! The only problem is that it somewhat lowers your relations with other states when you diplo-vassalize, so I actually only did it once or twice so as to keep relations with many HRE states very high; 150 or more is ideal.
Always accept royal marriages - especially within the HRE - to both indirectly (through improved relations) and directly increase the chances of Imperial Reforms being accepted. Also, try to create as many royal marriages yourself as possible. Marry anyone and everyone (within the HRE).
(Orthodoxy) Don't go more innovative until much later after you've passed the decisions that give you more missionaries (you'll be needing A LOT of them to convert Europe to Orthodox); staying at Bohemia's initial +1 means your tech isn't hurt TOO much, but you can quickly switch to +2 to get your extra missionary decisions, then start going more innovative after that.
(Orthodoxy) Don't adopt the NI Church Attendance Duty until you've passed the Act of Uniformity, as this NI stops that idea, which is a nice one as it gives extra missionaries.
Split that big Bohemian army into 2 immediately and move them to different provinces or it will take constant attrition; still, the Emperor Bohemia should maintain a nice big army always, in several stacks - as large an army as possible without requiring crazy minting and inflation for Bohemia; I suggest 3 stacks of 3 cavalry and 9 infantry each, for 36,000 troops total, though you'll probably have to lower Bohemia's military support limit to even break even on this most of the time. Bohemia is what I'd classify as a 3 on the 1-to-5 wealth scale; not dirt poor, but not filthy rich either. It can just about support a somewhat big army as Emperor, but try and field anything like your HRE Emperor force limits of 200,000+ and you'll be bankrupt overnight - it's no Burgundy (seriously, do they print all the world's money in Antwerp?) Early on in the game, many territories have a support limit below 12, so be careful which ones you step on so as to avoid nasty attrition where possible.
Don't forget to wait for 4 magistrates and expand the bureaucracy before doing anything else with magistrates or changing national focus (always worth it and I often forget!)
Declare a war of aggression with Muscowy on day 1, changing the sliders to max stability until you get to +3 (also, cancel Silesia's vassalization). After a month, declare on Silesia (reconquest CB) and annex them ASAP. Bohemia could do with being a bit bigger to support the large army. In my game, I also took a couple of states off of Hungary for 8 badboy, but this was probably unnecessary.
As soon as possible, "lose" the war (haha) with Muscowy by offering tribute and converting yourself to Orthodox.
Get a level 6 diplomat as soon as possible so that, combined with your diplomatically-mediocre leader, you can lose nearly 1 badboy per year. Get to zero infamy as quickly as possible, and stay there.
Now, enforce religious unity on at least 3 of the electors; the smaller/weaker they are, the more likely they are to accept. Maybe try Saxony, Mainz, and Trier.
I won't lie, this is where some luck manipulation is needed in order to form the HRE super-fast. You won't get a 'yes' every time, and going to war to enforce religious unity is slow, painful, and ruins your relations with all the states who enter the war. When it came to enforcing religious unity on states, and proposing imperial reforms, I may have taken steps to increase their chances of succeeding - royal marriages, improving relations to over 150, allying, maintaining zero infamy, etc. - but in the end I just saved and reloaded until the decision was successful. You can't get things done quickly otherwise. You generally want the chances of success to be at least 'Unlikely'; as you go on and improve relations further, it will often be 'Maybe'. That'll only take a few rolls of the dice at most.
If your leader dies before this point (don't make him a general - that increases his chances of dying) and Austria or Burgundy become the Emperor, you're screwed; try again.
Here's the great part - you
don't even need to vassalize any electors in order to basically guarantee re-election from now on! The Orthodox electors
cannot vote for heretics (ie. Catholics), so as long as you don't get any large states such as Burgundy, Austria, Bavaria, Milan, or Brandenburg to convert, the only choices they have are you, and distant second places who are either penalized for being too small, or not even being HRE members! No associated badboy for vassalizing electors is a major plus. I managed to get myself converted to Orthodox and convert 3 electors within 5 game years. As Bohemia is an elector and there are only 7 electors, this basically guarantees you'll always get a majority.
It's quite amusing to mouseover the electors to see their ratings, and see that the Orthodox electors' closest candidate to you is someone like Muscowy!
With permanent HRE Emperor status ensured, you can now get on with securing Imperial Authority and passing Imperial Reforms. When you get a NI (government tech 4), go for Military Drill so that your already-big armies will be even more likely to win pretty much any war you're likely to get into (stay stronger than Austria, at least).
Any extra cores gained from the "claims on our rivals" or "border dispute" events should probably be ignored until after the HRE is formed. Ideally you want to stay out of wars if at all possible until the HRE is formed; they waste time, and hugely damage your relations with anyone who gets into the war on the other side. You need to keep very good relations with the large majority of HRE states, so you never want to go to war with any of them, even on an Imperial Liberation CB or something (you'll still be against some HRE states, which will hugely damage your relations). It's even more important with this strategy to keep extremely good relations with other HRE states (ideally over 150), because when considering whether to vote for your Imperial Reforms, there's a small penalty for having a differing religion. Not too bad, but certainly not in your favour.
During the time you're converting states to Orthodox at 10 Imperial Authority per time, you should be systematically going round HRE member states and improving your relations with them. First concentrate on the ones that are already good, and get them to over 150 (check your country's relations list and order from good to bad). Then look at the ones below 100 and get them to over 150. Then, try and get them all to over 150. Also setup as many royal marriages as possible, and ally with many small states if it's convenient. Numbers of diplomats are always the bottleneck here, especially as you lose your +2 diplomat/year bonus that you get as Catholic, but you still get a decent enough number per year as the Emperor. If you happen to get into a war, that's +1 extra diplomat per year too. Maybe try and stretch it out for the extra diplomats.
Bestowing imperial authority seems to increase relations a bit quicker than giving gifts, but don't use that too often as you need most of the Imperial Authority for the reforms. Maybe use 20 to 30 Imperial Authority over time for increasing relations. You should also maintain a large sphere of influence (about 10 states) and hire a level 6 diplomat to greatly improve your diplomatic chances.
Asking for military access (and giving it, sometimes) is also a great idea both for boosting relations... and just to be able to move your troops around the HRE easily.
Don't worry so much about good relations with big states (6 provinces or more) like Burgundy, Austria, and Switzerland - they'll almost certainly refuse any Imperial Reforms (screw them).
Well, at least leave those guys until last when improving HRE state relations. In the end, they're all important really. Try to keep them over zero, at least.
Also, don't enforce religious unity if your Imperial Authority is already at 91% or higher; it maxes out at 100%, so either bestow some Imperial Authority on improving relations, or use it to implement an Imperial Reform.
You can try to implement an Imperial Reform when your Imperial Authority is at least 50. However, ONLY DO SO when Imperial Authority is at 51 or higher (except for the very last reform, forming the HRE state), because you need at least 1 Imperial Authority to execute the 'enforce religious unity' decision and get more Imperial Authority.
If the reform fails, honestly, you should save beforehand and load and re-roll the dice. Look in the log at who voted in favour and who against, and count the for/against numbers. If it's close, re-roll the dice a few times. If not, butter up some more states to love you and vote in favour. Note down a list of non-large states who voted against. Some will just be bad luck, but in most cases you can go down that list and improve relations with that state to over 150, as well as forming a royal marriage.
Implementing reforms should get easier over time, as you're converting more and more states to Orthodox so you don't get the "different religion" penalty. After a few successful reforms, the rest should just all pass (assuming you stay at zero badboy and don't get into wars with them). Nevertheless, ensure very good relations with all states (small and large) so that the penultimate reform doesn't result in any states going to war with you.
You pretty much want the entire HRE to be one big incestuous lovefest by that point.
Finally, enact Renovatio Imperii and form the HRE! It'll be an Orthodox Slavic state with its capital in Prague... not quite the characteristics typically associated with the HRE.
(Orthodoxy) You want to get those extra missionaries to convert tons of Catholic territories to Orthodox, but some are only available as a Catholic state. To temporarily be a Catholic state, "lose" a war with a Catholic state, much as you did at the beginning with Muscowy, and offer them tribute, converting yourself to Catholic. Once you get the decisions for the extra missionaries, "lose" a war with an Orthodox state to switch back to Orthodox. The same method can be used to convert temporarily to Protestant/Reformed.
(Orthodoxy) Wait until government tech 15, get the innovative slider to +2, and pass the Conventicle Act and the De Heretico Comburendo Act (need to be Catholic).
(Orthodoxy) Conquesting Jerusalem and Mecca is also a good idea for more missionaries.
(Orthodoxy) Other good decisions for religious conversion: Establish Sunday schools, Act of Uniformity (must not have the national idea Church Attendance Duty), Advancement Of Religion Act.
(Orthodoxy) For even MORE missionaries, wait until government tech 21, and pass the Test Act and the Popery Act (no slider requirements, need to be Protestant/Reformed).
