You can fuse by being the same race and having earrings or both have the skill of "fusion".
While Battling in a fuse the person with the higher power level controls the person. But if the person wants to let the second person battle for a bit he can. The power level addition is just for the fusion. Also, when you come out of the fusion you perminatly lose stats.

Saiyan Fusion:
Both players add half their power level to their power level then they both add their stats together. EX. 10,000 pl wants to fuse with another 10,000 power level. They both add 5,000 to their power level making it 15,000 then combine it for 30,000.

Namek Fusion:
Both players add +250,000 to their power level then add their stats together.

Android Fusion:
You add your stats together. Fusion kills the weaker android after fusion is broken.

Other Fusion:
Both players add their stats Together. Ex. Just add like a power level of 10,000 adds to a power level of 20,000 makes it 30,000.