I don't know if this is the best way to do it, but this is how I've done the three times I have used gelatin to clear a beer.
The day before adding gelatin I cold crash the beer at around 35-40°F.
On the next day I take 1 tsp in little less than a ½ cup of water for a 5 gallon batch. I heat it gently to 158°F (stir while heating) to sanitize the gelatin, cover it and let it cool down to room temperature. Then I poor it into the fermenter and stir gently to mix it into the surface.
1-2 days later the beer is clear and ready to be racked to a keg.
Once I was clearing a 10 gal. batch split in two buckets. I didn't pay attention while heating the gelatin for one of the buckets and the temp rose to 170°F. That bucket did not clear as fast as the other bucket where I only heated to 158°F so I guess you will "destroy" the effective parts of the gelatin by heating it too much so the clearing I was seeing was from the cold crash.