I have not used gelatin yet but I intend to this weekend. After reading several threads, the most often referenced procedure is from BierMuncher in the 'gelatin finings?' thread. Here it is:
I use a tablespoon per five gallon batch.
Mix it with hot tap water in a sauce pot, about 1 cup of water per tablespoon.
Stir it up and let it sit for 20-30 minutes to hydrate and bloom.
Put the pot on the stove and heat until it looks like it’s about to start boiling…don’t boil.
Cool slightly (I put my pot in a cold water bath).
Add it (gently) to the secondary (or keg) as you’re racking your beer.