Open map you like and learn :)) There is not much of forest possibilities anyway, like for example you have 3 types of pines and 3 types of pines big chunks. Also these big blocks of forest are problematic, they take more area than visible, and when artillery hits them they disappear as whole. I use them only on areas far away from roads and resource points.

It's all personal preference, but I find the pine trees my favourite. 3 single pines under 'small trees' and '3 others under large. Dont use 'simple' unless off map.
Always use random rotation when playing them.
You can also vary the height by small amounts to add more variety.
I find the bushes and scrub go well with the pines too.