I have a few central hill pieces that I use. They are a combination of modified store bought and scratch built pieces. I think this is a good approach, especially when starting a terrain collection.
These first 2 pieces are a set of store bought injection molded foam rock outcroppings. There was originally very little "ground", and what there was, ended abruptly in a step. I personally prefer a gradual slope to get a more realistic look.
This image illustrates the parts that I added. I used 1" pink foam board and hot glued it to the hill. Then painted and flocked to match my boards. A top heavy mini will have an issue balancing on the slope so there is a trade off for the aesthetic.
All I did to this injected molded foam piece was to shave the hill level. Using a serrated knife (a Ginsu actually), I created two flat areas at 1" and 2" high. It originally was sloped all the way with no flat surfaces.
I have illustrated here where I leveled the hill. The left area is 1" and the right is 2" tall.
This piece was scratch built out of pink foam board. I thought I used 1" foam, but to my surprise (after the piece was completed), I discovered I used 3/4" foam instead. Actually, I didn't even realize I had 2 types of foam board!! I was not satisfied with leafing the hill "short", so I cut 2 pieces of cardboard and glued it to the bottom increasing the height to 1", blending in the details then repainting and flocking to get the final results (below).
Here is a top view of two of the hills stacked. As you can see I sculpted the lower hill piece to match up with the rocky portion of one of the
I used a combination of a hot blade and a heat gun to work in the rough rock detail. I have this really cool butane multi-tool that has different replaceable tips (soldering iron, heat gun, hot knife, and blow torch) The next step, I smeared spackle around with my finger and finally coated all the rock surfaces with watered down white glue mix.
The ground was coated with a sand mixture that I glued on. After drying overnight, I painted it brown and then dry brushed tan. I dabbled a little bit of sand and some larger granules around the rock surfaces. The rocks were painted black, dry brushed with a cool-gray, and then washed with a greenish-brown mixture.
I use inexpensive craft paints for my terrain. But I did buy a large bottle of scenic paint that I use as a wash to "dirty" things up.