Depends what sort of game you're playing. It's expensive, and if you do a lot of conquering it's hideously expensive.
You tot up the +ve and -ve relations of each country which owns provinces inside the HRE outline, with every Catholic/CRC nation in the world (including itself, if it's Catholic
the nation with the highest net relation gets the Imperial Crown. All things being equal, it tends to be Austria. If you own parts of the Empire, having a negative relation to yourself (which you get by annexing stuff) actually hurts your chances, because your own votes go to other people.