Luke warm water. Depends on age. And enclosure. How high do you have the humidity. I soak 3-4 times a week. But my humidity is at 80-90%. And they get misted down 2-3 times a day.
Pretend you are about to bathe a baby - comfortably warm water for 10 - 20 minutes.
Just enough water to come to the edge of the shell - where the top meets the bottom.
Stay near by and keep a close eye in case of climbing or turning over or you never know what all.
Feces are a given; I like to clean it out of the water so they don't have to swim in it and . . they might want to drink.
Rub-a-dub-dub
Aim for about 90-95F for water temp. Soak as much as is needed to make sure he is well hydrated. I usually inspect for injuries or anything unusual during these times, too.