It's very easy really, Just open the hpl level editor and mess around with it!
Just make it look like an outside area that's all... here is an outside maps I did with the hpl level editor!
Pick a dirt plane and put some trees around, some fog and lighting with some plants and there is your forest map.. pretty easy question to answer really! if you don't know how to use the level editor you could always check YouTube tutorials of the level editor here
https://www.youtube.com/watch?v=2FR4p5tt0BY
Goodluck my friend