I've read a lot about gelatin on these forums and these 2 methods sound the best to me. I use Method 1 pretty often but can't vouch for Method 2. I get LD Carlson gelatin from HBS, it's cheaper than buying at food store.
Method 1 (similar to what's on LD Carlson bottle)
Use 1/2 teaspoon per 5 gal beer. Mix it with hot tap water in a sauce pot, about 1 cup of water per 1/2 teaspoon.
Let it sit for 60 minutes to hydrate and bloom.
Heat the pot 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. Some say the beer should be cold first, some say you add the gelatin and then cool to fridge temp. I've done both but am not sure which works better.
Method 2 - Use a glass jar, and fill it with a cup or cup-and-a-half of tap water. Not hot or warm or cold.... just cool. About 70-80 degrees. Put 1-3 teaspoons of gelatin in there and let it sit for 20 minutes. Swirl it up good (it will be cloudy) and pop it into the microwave. Set it for about a minute, but you need to watch it and take it out as soon as it's clear. The glass will be warm. If you try to dissolve it in hot water, it clumps up, and cold water won't soften it up like you want. Never boil it.