First off let me say that the preferred method of encouraging nesting is outdoors. It is very difficult to get conditions absolutely correct indoors, and if you want a tortoise to nest you must make every effort to do so.
In order to encourage nesting a few criteria must be met. These include a gravid tortoise, soil consistency and moisture, soil depth, soil temperature, solitude and a willing tortoise. I have found that the best set up to encourage nesting involves a soil container sunk into the floor of the tortoise's normally occupied habitat. The reason for this is that often tortoises that are not in their normal surroundings will not lay. When the tortoise is not looking for a nesting site the soil container can be covered with a piece of wood to make it easier to keep the habitat and the soil clean.
It is difficult to explain how one knows that a tortoise is ready to lay. Experienced keepers often call it "walking the walk". The female begins to pace her habitat as if searching for something, occasionally sniffing the ground as she walks or trying to escape by climbing the walls. If she has other tortoises in with her she may seek to exert dominance over them, often ramming, biting or actually mounting them. It is at this point that you must present her with the nest box.
The soil should have the consistency of good friable garden soil or potting soil. It should not under any circumstances contain perlite as a gravid female tortoise has a heavy calcium hunger and will eat anything white. When you pick up a handful of the soil it should clump easily without being muddy. The female needs to make a flask shaped nest to deposit her eggs in. Soil that is too dry will not allow this and she is likely to give up. If the soil is too wet she will not even attempt laying.
The soil should have a depth of at least two thirds the length of the tortoise's carapace and preferably the entire length of the carapace. If the female hits the bottom of the nest box she is likely to give up as this is similar in nature to encountering a rock. For my nest boxes I use the largest plastic mixing bowl I can find or a wooden box constructed to size.
I strive to achieve a surface temperature of just about 95 degrees F (33 degrees C) and a temperature two inches down of about 84 degrees F (29 degrees C). I do this with the use of a clip-on incandescent light positioned above the nest box.
I have found that a watched tortoise never lays. They need absolute privacy. In nature a nesting tortoise or her eggs are an easy lunch for anything that comes along. If she is distracted she will not lay. This includes the presence of her mate and other females. They must be removed for the duration. In contrast once she has actually begun her nest she may become oblivious to watchers. She may dig more than one trial nest before actually laying or even stop looking for a site for a week or more before she starts to seriously look for a nest site again.
There are occasionally tortoises that will refuse to nest indoors under any circumstance. When this happens there is not a lot you can do to encourage her. I would suggest adjusting the soil temperature, trying two or three days at 105 degrees F (40 degrees C) followed by two or three days at 86 degrees F (30 degrees C). Another practice that sometimes helps is daily warm soaks in shallow water. If at all possible setting up a warm spot outdoors can be tried as well. If the female still refuses to nest one of two things will happen. The first is that she will lay her eggs on the surface or in her water dish as she walks around the habitat. In this event incubate these eggs as you would normally. The advice often given that surface laid eggs are always infertile is not true and eggs laid in water, if caught in time, can survive. There is a chance that the female may become egg bound. If she develops a noticeable limp, gets lethargic or loses the use of her back legs there is no recourse but to get her to a vet. .
After successful nesting you may carefully dig up her
eggs and after marking them to ensure proper placement, place them in an
incubator to await your hatchlings.