I put the rat(s) in a plastic baggie, put scolding hot water in a container, and put the rats in that container of water for about 30 mins - 1 hour depending on the size of the rat
To prevent "cooking" them I turn them midway through, or if they are thawing slowly I will take them out of the hot water for like 5 - 10 minutes
Also keep in mind I constantly feel around the rats guts to make sure nothing is still frozen, and I also watch the skin to make sure it doesnt show signs of cooking
(A cooked rats skin will usually turn pink/redish, not good lol)
So yeah thats pretty much my OCD of making sure my snakes dont get a frozen/cooked rat summed up in one post LOL