The base map in the scenario editor begins as a giant green pasture, covered entirely with grass. However, good terrain design will help fix this. Begin by opening up the terrain tab at the top of the screen. Then you should see the list of available terrains.

The terrain in the previous picture probably looks bland. People wouldn't typically want to see this in a high rated workshop item. We should first pave a road between the trees.
However, this still looks quite boring, so in the next section we will cover how to add extra detail to this map area.

Now you need to learn about terrain mixing. Terrain mixing is a technique used by all of the professional map makers, both on the mod browser/workshop and in the official campaigns. The goal is to get dirt, mud, underbrush, and other terrain types mixed with your grass or other base terrain to create a realistic look. Lets start by fixing the road. The road visibly clashes with the grass, so lets add some dirt 1 along the edges.
As you can see, we also mixed some dirt and underbrush into the grass around the forest.

The time has come to add in some gaia objects. Gaia objects include bushes, flowers, statues, ruins, waterfalls, and the grass patch. Gaia objects can be dangerous for inexperienced designers, as many people use too many of these objects, especially grass patches, as a way to cover up bad looking terrain. With that said, click on the others button at the bottom of the screen. A list of gaia objects should pop up, assuming the selected player is set to gaia.
Now lets add some bushes along the edges of the forest, along with maybe one or two by the road.
Scroll down the list until you find rock 1 and rock 2.
Then place some on a few different edges of the forest.
Next lets add grass patches. Scroll down the list until grass patch pops up.
Place some on the outskirts of the forest, and maybe one in another area.

Finally, lets add some final scenery objects. In this part of the map, I added ruins. Along with some berry bushes and mines. Simply pick what ever goes best with the theme, and your personal tastes. After this is done, your forest scenery is completed.