22 -24 days after BREEDEDING occurs they lay (approx) now if you do not know when /if breeding occured, you still must have the nest, she will lay eggs anyway (some re-absorb so if you never see any after a long time thats why)just added that in as people sometimes wonder after months WHY and waht happened to her eggs she did not lay and now is looking normal again.
Okay, yes put your nest container in a corner or I use different methods, 2 bricks in an L shape in the corner back of tank and side of tank make a total square, leave an opening/door for her to get in and fill with about 3 -4 inches of dampened sand more if ya want. You can put a removable top on such as a small piece of slate. wood, even a firm book works.
* I have also used 2 rocks kinda stagered (not lined up and then put a piece of slate over.
* A new nest I have used is a medium terracotta pot broken in half, (easy to look in and see whats happening. As long as they feel secure and know they have a secluded spot most will use it.
There are that few (I have some) that do not use anything you make for them, they just lay where they choose, 
can't argue with an egg laying Mom, ehhehehee in that case just wet some sand here and there if you notice she has picked her own spot.
She will begin digging near her time, digging non stop you will also notice she goes from a famished appetite, to no appetite the closer she gets to laying.
As far as an incubator, I use the hovabator incubator about 34.00 bucks, sometimes you can find one on ebay ....
OR you can just have a plastic shoe box sized container, fill it with dampened vermiculite, put a small hole in the top of container about the size of a quarter.
Put the shoe box/eggs in a place where it will not be cooler/drop below mid/high 70's at night and not go over 90 during the warmest part of the day.
I think thats it if I forgot to answer something , I will when I remember. 
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