How to set up a turtle egg laying place?

Okay, so maybe about 2 months ago I saw my 6 and a half year old red eared sliders tickling each others face (there mating sign) and now the larger one (the female) has been trying to get out of her cage, not eating and i'm pretty sure she's pregnant. I'm scared because about a week ago while my family was in vacation my sister said it got out (which means it had to of hit the ground pretty hard, its tank is about 3 feet off the ground) and i'm scared for her and her eggs. I have an old 20 gallon tank and today i cleaned it out, and i was gonna get some plant soil or much today for it to have its eggs in. It keeps trying to get, i'm guessing because its tank is water and doesn't want to kill its babies? Anyway, we're using the other light to put over the eggs, what else do i need to do to keep her and her eggs as happy and healthy as possible?