These hills were made on 2 foot by 2 foot sections. I keep the bases of the hills away from the board edges so the table can be set up in different ways.
If you are making high hills as a one piece construction. i.e. one big hill.Give your men a way to get to the top. I like using a combination of natural steps with gentle slopes. The effect looks good and it is quite easy to do. I start at the bottom of the hill piece and using the wood rasp, I just start filing steps up one side. It does not have to be neat, you want it rough looking. Just make sure the men will have enough room to stand on the step and remember the "gravity/erosion" characteristics of nature .
Higher hills are great for ravines and water falls too. try to work one in to a cliff face.
I try to put another way up the hills too. Kind of a secret trail from the back side. Makes for more variations in the game.
Notice how the undercuts and cliff faces "do not have green flocking". And when it does it is only on the "top" faces of the ledges. (when adding it it is dropped straight down while the terrain piece is flat) Gives it all a more natural effect.