STEP 4 - WATER BODIES
Create a new layer and rename it to "Water" or something like that. Put it below your "Land" layer.
Highlight the "Land" layer, use the Magic Wand on "Global" mode to select all water regions. Now switch to your "Water" layer, select a good water-y color (I'm using #0026FF) and hit Backspace to fill your water bodies. Next, switch colors and select a darker color to go with your primary color (make sure the lighter color is primary and the darker color is secondary). Now, apply a Clouds render to your selection, using Scale "1000", Roughness "1", and Blend Mode "Normal". This should make your map look like this (only the "Water" layer visible).
Your next step is using the Fragment effect with Fragment Count "3", Distance "3", and Rotiation "45". After that, add some Noise (I'm using Intensity "50", Color Saturation "0", and Coverage "50"). Finally, use Gaussian Blur with Radius "1". Now, with all layers visible, your map should be something like this.
STEP 5 - FORESTS
Create a new layer and rename it to "Forests" or something like that. Put it between your "Land" and "Outlines" layer.
Switch to the "Land" layer, and use the Magic Wand on "Global" mode to select all landmasses (by this time you probably would have to select all water bodies and then invert the selection with CTRL-I). Now switch to Lasso Select, change the Selection Mode to "Substract", and cut out every piece of land you wish not to be covered by forest. Switch to your "Forest" layer, select a forest-y color (I went with #228B22) and hit backspace to fill the color in. You should now have something like this (only "Forest" layer visible).
Next, add Noise (Intensity "100", Color Saturation "50", Coverage "50"), apply a Fragment effect (Fragment Count "3", Distance "3", Rotiation "60"), and finally do an Ink Sketch (Outline "40", Coloration "100"). This will get you something like this (all layers visible again).
STEP 6 - MOUNTAINS
This section of my tutorial is based on this tutorial for drawing fantasy maps with mountains. I had to adopt it to work with PDN, but the basic idea is the same. Anyway, let's go on...
You guessed it, didn't you? Create a new layer and rename it to something like "Mountains". Put this layer either above or below your "Forest" layer (you can later switch the position if you like it better one way or the other).
Use the Lasso Select to select an area where you want a mountain to be. Switch to Paintbrush and leave the brush width at "2". Use black as your primary color. Now draw your mountains main ridges. Then change the brush width to "1" and draw some secondary ridges. You should now have something like this (only the "Mountains" layer visible with the current area selected).
After doing that, increase the brush width to "3" (or even larger, dependent on the scale of your mountain) and switch to a dark grey-ish color (I used #404040). Paint the shadow areas of your mountain with the brush. Be careful not to completely draw over the black ridges. If you do, just hit Undo and try again. Then select a light grey-ish color (I went with #808080) and paint the other side of your mountains. You don't have to completely fill the space, some white specks and spots are completely fine. Now your mountain should look like this (same visibility as above).
Having done that, change the brush width back to 1 and paint some strokes of light grey in the dark areas and some strokes of dark grey in the light areas to simulate smaller ridges and valleys. If you have finished this, carefully paint some black accents on the dark grey lines and some white accents on the light grey lines. Your mountain is almost done and should look like this (same visibility as above).
Now, as a final step, use Gaussian Blur with Radius "2" on your selection and after that apply Ink Sketch with Ink Outline "25" and Coloration "25". Now your mountain's pretty much done and should look like this (all layers visible again).
As an optional step, you can apply a Clouds render to your mountains to randomize the shading. Select the light grey as primary color and the dark grey as secondary color. Then do the render with default settings for Scale and Roughness, and Blend Mode "Overlay". Just hit Reseed until you like the shading. You've just completed your first mountain mountain (all layers visible).
Repeat the process for any additional mountains.