There are plenty of articles on how to make hills. Here is mine:
I have seen many great articles on making hills but none of them appealed to me for various reasons.
Due to limited storage area I needed hills that were light and durable, slightly flexible would also be good..
I was making some native huts for a colonial game and was thatching the roofs, as per an article I had seen on Terragenesis in which the "guy" had used a terry cloth towel as thatching, after I thatched my roofs I still had a large piece of terry cloth towel left and thought why not try flocking one of my upholstery foam hills (which were functional, but didn't look that great). I dug out my hot glue gun and glued it on, it looked a bit better (colour kind of sucked), then I brushed on a 50/50 mixture of water and PVA glue completely soaking the towel, let it dry, painted it green and dry brushed it in lighter greens and yellows and it looked 1000% better.
It is quite durable, as a demonstration I grabbed one by an edge in both hands, raised it over head and banged it onto a table corner repeatedly, no cracking chipping or damage of any sort, the white glue and the cloth are stiff but still flexible.
Step 1: Cut a piece of upholstery foam into a hill shape, angle the sides.
Step 2: Hot glue a terry cloth towel to your hill shape. Take care to glue at all edges near the bottom.
Step 3: Cut around your hill leaving a bit (about 10mm) of your terry cloth towel extending beyond the base of the the hill.
Step 4: Mix white glue and water 50/50 in a container, and using your bigger paintbrush paint it onto the terry cloth towel, better too much than too little. I added some green paint to mine so I could see what I had covered as my terry cloth towels were white, if you start with a dark colour towel you won't need to add paint.
Step 5: After it has dried (mine took a bout a day) go around the base of your hill again with a pair of scissors trimming the excess towel away (the glue reduces the chance of the terry cloth towel unravelling), paint it green or brown or both then drybrush.
Here are the some hill in use with different scales. The British and Zulus are 1/72 scale (20mm), whereas the Stormtroopers are 30mm.