Concepts.
We know that, nature/forests are almost random, yeah...
But, we are making a map in a game, games are not totally random, so why are we going to put trees all randomly anyways.
Of course we can put them randomly, but, if you go too randomly, it will look worse.
Trees are quite the most common thing coming to our minds when mapping a forest.
Sure, they look great, but, if you overload the map with them, it won't look quite good.
[Show/Hide] Spoiler.
==========================================================================
STEP 1:
Before you start with your map, you may want a building or 2 in the forest, you know, a sign of someone living there.
I think it is better to start first with the building, in the blank forest... Remember to take your time for making the building.

STEP 2:
The entrance, mark the entrance with an auto tile, and then detail it with trees, flowers, rocks, and even water.
Note that, the plain area with nothing but floor is blank because I still not map in there.
The grass auto tile is quite useful and looks really nice, but if you cover the whole place with it, things won't look so nice, so remember to keep a few blank spaces, where you can put other stuff.

STEP 3:
All the other place may be filled with the same kind of stuff.
The idea is to not repeat the same thing too much, if you do, do not make it too near of each other.
The building is to the upper right, don't worry about it.

I would like to add more information, tips, and so, but I got to go right now, I am not really good mapping anyways
Thanks.
Omegas7.

