you dont really get them to do it they know on their own when they are ready. what you do need to do is provide a laying bin. ( container with 8-10 inches of washed damp playsand )
jannb has a great blog on laying bins.
often you can tell they are ready when they start searching the bottom of the cage and often slow their eating.
its a good idea to keep a bin in there at all time so she can dig if she wants