That's a couple of good questions. Yes, you can warm the milk. Or if you are using whole milk you can add some hot water but make sure you use mostly milk. I wouldn't recommend 1% or 2% since it's already mostly water. The Amish actually use the milk straight from the cow. Of course, how many of us actually have a dairy farm? :)
It's hard to suggest how much to use since you can actually soak in different places. Just make sure that when you soak the entire rectum is covered. Just like you would in the when taking a bath. It takes patience and soaking.