There are many ways in which you can set up your ant farm,you can mix up some grout get it as close as you can, to the shape of your enclosure, then carve out the nesting chambers, I use this method but I also use the easier method which is sand.
You will find out on the internet a lot of information about this differant keepers may have their own mix, which they swear by, the general rule is, if it is working for you, then keep using that method.
I use either playsand as it tends to be dense and holds it shape when damp but I also use a sand and light clay mix for extra stability.
Fill your ant farm with the mix and make sure that it is damp, do not fill your farm to the top as the ants will need to hunt on the surface,when happy with the level use a toothpick or wooden skewer to make the starter tunnels.
This will encourage the ants or queen to use those and complete the job, don't forget to cover the back of the farm where you ahve made the tunnels with black card, othwise the queen will dig away from the light.
Below shows an example of using a grout mix once mixed and set, it is carved to the required style, the photos below show the finished nesting area, it is wrapped in polythene to stop the sides of the farm getting scratched,before the final fix.
Once happy with the fit the polythene is removed before the nesting area is fixed into place with aquarium sealant
The second photo shows an all important way of getting water into the nest with a fixed cap to prevent drownings