For the skill-building layout for my son, we used the old technique: Vertical wooden posts, a network of masking tape, wadded newspaper on the tape, paper towels laid over the newspaper and wet down, then plaster-soaked paper towels on that. Once that dried, we did a second layer of towels. Then you can shine a light from underneath (after removing the tape and newspaper) and see the thin spots, to add more plaster-soaked paper towels.
You can buy plaster at Fleet Farm or Home Depot. IIRC, the ratio is two parts plaster to one part cold water. You have to mix the plaster in relatively small batches, since it sets quickly. Your fingers WILL get messy. The floor will also get messy, so lay down some thick plastic.
The weight of the plaster-soaked towels will change the shape you got with the wadded newspapers. You can add more newspapers on top to create another bump where you once had a concavity.
The bottom line is that it is really messy. Wear old clothes you don’t care about and protect ANYTHING you do care about. Remove rings, tape over the track, etc.
The result is also HEAVY. Make sure you have good benchwork, or you will find your legs bending when you try to shift the layout. It may take two or more people to safely move the layout (if it is remotely moveable), even a small one, because of the weight of the plaster.
Also, if you plan to have a fascia panel, install it first, screw in wooden blocks (blocks on layout side, screws counter-sunk on aisle side). Make sure you wrap a plaster-soaked towel around each block, and then plaster right up to the fascia. When the plaster is all dried, pencil a line for the profile of the plaster scenery. Unscrew the fascia, use a jigsaw to cut the line, now you have a nice fascia cut to follow the landscape.
I picked up a hand-held sprayer a few years back, it holds about two gallons of water. it’s the sort of thing people use for insecticide, but if you have only ever used water in it, it is