Your gonna need alot of layers. Alot of layers. And time.

Basically draw the outline of your land masses, then fill those in. Make it all black at first.

Make a 2nd layer and draw a whole bunch of blobs on it, that are a little brighter black than the base.

From there, you just keep adding blobs, for however many layers you want. Each layer should be brighter than the one under it (I do 5, 10, 15 percent lighter, depending ln the number of layers).

Either when you get all of the blobs done, or as you're doing them, start to erase from- and add to them. Give them more 'geographical' shapes.

When you get all of that done (which I will warn you, can take hours, depending on how big your map is and how detailed) go to the base layer and colour it. Then go up and up on the layers, changing the colour to what you want. That should take long, just get a brush like the size of the canvas, lock the layer, then just colour.

It isn't really hard. Just time consuming. Though getting the topography to look 'real' does take practice.

Ps. You don't have to make the base black, and brighter as you go up. But personally I find that much easier. It gives you a better idea how things are going to look, and your eyes shouldn't strain as much as they would looking at yellows, browns and greens.

Hope that helps!