I can't immediately think of any guides on forming the HRE. I have a thread one becoming Emperor, but I assume you already are emperor.
As a quick summary, you need Imperial Authority. You can get that by releasing nations into the HRE, i.e. creating new princes. So find instances where there's a non-existent nation whose cores are in HRE territory, and DOW the current owner of those provinces and force them to Release Nation.
Winning any Enforce Peace war as Emperor will give you 20 IA - even if you Enforce Peace against an attacker who is not in the HRE and is not even fighting the HRE! That is one of the largest IA boosts you can get. So find a war you can win and where you have or can get +100 relations with the Defender, then Enforce Peace on the attacker.
If you're an HRE member and you have any provinces that aren't in the HRE, add them, to gain +1 per province
Here's more details on what gives you IA:
http://www.eu4wiki.com/Holy_Roman_Empire#Imperial_authority
You can pass a reform once you have 50 IA. For the first 6 reforms, all you need is 50 IA and a majority of princes to agree to them. You might need to boost some relations, or Bestow Imperial Grace, to get enough votes, but it's not too hard.
For the last two reforms, you really want 100 IA, because you want every prince to agree to it. The 7th Reform will turn all HRE member states into your vassals. Any member that votes against it will leave the HRE. The 8th reform is the formation, and again any member who votes against it will leave and you won't get their land (you're meant to get a bunch of cores but that is currently bugged in 1.3.) It's easy to get all princes for the last reform, because by then they're already your vassals. So it's the 7th that really matters, and really the best way to ensure that is to get 100 IA and therefore you're going to be needing to do lots of IA boosting activities, which primarily means Enforce Peace wars and trying to add more nations where possible.
If you have non-HRE vassals, like for example France or Castile, one trick is to integrate them, then add all their territory to the HRE (+1 IA per province), then Release them as Vassal for a further boost of up to 20 IA for creating a new prince. That also turns them into a free vassal once you've passed the 6th reform, Erbkaisertum, which turns all your HRE vassals into privileged vassals meaning they don't use a DipRel slot.
So if you have, or can get vassals on the borders of the HRE, it's a good IA booster to integrate them and then add their lands and release them as HRE vassals.
Finally, make sure you never decline a CtA from an HRE member! That will lower your IA.