Hills

Add 2 new layer folders (not sublayers!) above it.

Change the name of the layer folder to  "Sky". THen rename the other layer folders -the middle one "Hills" and the top one "Trees". 

Lock "Sky" and "Trees" folder. Select the layer folder "Hills" and insert a new bitmap layer so you can draw. 

Set the Brush tool to Basic>Hard Rounded type, medium green color, very big size (around 100), no softness for the Edge.

Draw the first hill...

Change the Brush color using the Color Wheel to a darker shade of green.

Insert a new bitmap layer and draw in the second hill.

In the Layers panel, click and drag the hill layer lower so it's below the first hill layer...

Insert a new bitmap layer. Make the Brush a darker shade of green using the Color Wheel, and draw the third hill...

Then click and drag the layer under the other 2 hill layers...

If you want, you can use the Eraser tool to erase parts of the hill you have drawn to shape it. If you do this, make sure the Eraser is set to a hard edge in Properties.

So you have something like this...

Add darker shadows near the tops. Select the Burn tool. Go to the Bitmap tool section and hold down the mouse to get more options...

Go to Properties and set it so it's big, soft edge; experiment with exposure values (try out anything from 10-50).

Select the first hill layer. Click and drag the brush along the top edge of the hill only. If it doesn't show, your Exposure level is too low (undo and increase and try again).

To burn the second hill, make sure it's selected in the Layers panel.

Repeat for the third hill and you end up with something like this...