Here's a page on things you need to know:
http://www.redearslider.com/reproduction.html
Read it carefully, then look at some of the links at the bottom of the page. They have more info on nesting and how to prepare a turtle nest.
People on here with gravid turtles seem to have the best luck when their turtles can go outside to lay. They prefer somewhat softened dirt - like in a flower bed - in a somewhat sheltered location - like under some bushes. Sometimes a turtle goes through several false starts before actually laying. They'll get all restless, maybe even make some digging motions with their back legs, then seem to lose interest. The next day, they do it all over again. Finally, the day will come when they will actually lay, then you can breath a big sigh of relief.
After laying, a female will need to rest and will need extra calcium in her diet, so make sure cuttlebone is always available.