These hills are made from carved slabs of insulation board. You don't need to be that precise and you aren't really looking for a clear slope up every side of the hill. Some of the approaches to the hills can be sheer and others might be a more gradual climb for the figures.
Mark out your base level with a pen on the insulation board and then cut diagonally into the board. You can always trim the edges a little more if you think the slope is too severe. I went for four to five levels for the hills, each of the levels are around an inch or so.
I stuck the levels together with a hot glue gun and then started applying a creamy mix of wall filler. You can deal with any gaps and holes by pushing small stones or pieces of gravel into them.
Next job was to give the whole thing a thorough coat of PVA. The hills were then covered in a sand and gravel mix and left to dry.
I then gave the whole hill a good coat of brown paint to match the terrain boards. Two dry brush layers of caramel and then white were added. I gave the stones and gravel details a slightly heavier white coat to pick them out.
The final job was to add some static grass here and there again to match the terrain boards.
One of the hills before the static grass was added.
You can see that the static grass has a real impact on the overall look.
This is a flatter hill using a number of insulation board offcuts to give it an undulating look.
Another view of one of the hills showing all of the stone detailing picked out in white.
Two of the hills configured to be used as the Round Tops for Gettysburg.