Pretty much I do what Hammy suggested. If you are worried about using sterile water then pre-boil a cup of water or so, and let it cool. Toss in the Knox, let it swell, then heat the mixture up to 170F.
I can't claim to know the chemistry, but 170ish seems to be a temp you do not want the gelatin to get above. If it gets too hot is is "denatured" or broken down somehow and does not work at all. Thats the consensus anyway, and I have followed that rule of thumb.
Once it's up to 170, just slide the mix on top of your keg or carboy. It works.