Subs in HoI2 and DD are extremely different in terms of their capabilities. In DD, subs are very poor at actual naval fighting but excel at sinking convoys and escorts. In HoI2, it's basically reversed although they are okay at sinking convoys.
The basic gamey sub strategy for HoI2 is to make uberstacks of 30 subs under a good seawolf commander. These will sink anything and everything with very few losses until late in the game. Once the allies get their later naval doctrines researched you will start taking more losses. This is highly ahistorical and unrealistic, but it works and you can sink the entired combined allied navies with 90 subs or so.
A more realistic and historically accurate approach is to follow the advice as per Blue's thread in the DD forums. Just don't expect as good of results. Also, smaller stacks of subs will tend to get chewed up more rapidly in HoI2 than in DD if they do engage enemy forces, although they are more likely to take some of them with them.