This is the point I decided to glue the house to the base, and I cut out a bit of wood to extend the canvas as it slightly overlapped.
To create the base I squashed up tissues and hot glued them to the base to create the rough shape of the hills. This is to prevent applying the Fast Drying FastMache too thick, as it would take a long time to dry and would likely warp the base quite severely.
With the base shape applied I mixed water with the mache to a point where there were no dry areas but it wasn't overly wet, and then after applying blobs of glue like with the plaster, I used the flat putty knife to apply the mache all over the ground over the tissue hills and the base.
The main idea of this step is to add some texture and variety to the ground so it's not dead flat to add a bit of character to the piece.
I kept each of the corners clear so that weights could be added to each to try and prevent warping as much as possible. I think it likely only needs to dry for 12-24 hours but I left it for a week just to make sure. It still managed to warp a bit, but you can carefully bend it back into shape, using superglue to fill in any cracks.