HOW TO MAKE YOUR OWN TERRAIN:
Forests

• Felt
• Green and brown colored markers

Your troops must be able to march through the woods, so a flexible solution is called for. You need something to define the boundaries of the forest.

The easiest solution is a piece of felt, cut into large irregular shapes. These pieces of felt are put on the gaming table and define the borders of the forest. Trees are placed on the felt and can be moved around or removed, should troops wish to march through them. Use some green and brown coloured markers to vary the colour of the felt a little.

• Lichen or plastic trees or bump chenille

1. OK - you've already taken the first step by using green felt for the playing surface.
2. Lichen is a good move for forests. To make your woods on the table, you simply throw a few clumps of lichen on top of each forest felt shape. You now have clearly deliniated forest areas with the lichen for the 3-D effect. When troops move into the forest, you place them on the irregular sheet of felt and displace the lichen around the figures so that the minis are not standing "on top" of the lichen.
3. A more expensive method is to buy a few plastic trees from a railroad model shop (bring a figure with you to the store and compare the size of the figure to the size of the various tree scales to find the size you think looks best). Mount these trees to small bases (for added stability). Your forests now even look more realistic and you can still displace the 3-D bits to accomodate your figures without disrupting the borders of the forest.
4. Or, you can go nutzo with modeling techiques. I take bump chenille stems and melt them with a candle to make a trunk. Then I use glue and flock to make the top of the tree. (This is the GHQ Terrain Maker method of making trees.) Afterwards, I mount the trees on pieces of plain floor tiles that I've flocked.

Most people never go to the bother of the last step, and you normally don't need to (depends whether you are more interested in either how the game plays or how the game looks).