Now that we’ve got a good shape, now’s a good time to merge the threshold and cloud layers together. Then we’ll make a new cloud layer and another threshold layer above the continent. Merge the new layers together. Now we’ll use the lasso tool to select interesting-looking bits. Cut those out (hotkey Ctrl+X; see step4).
Hide the threshold/cloud layer (now dubbed Threshloud, a 14th-level Photoshop layer) and paste the little land-bit above the continent. Set the layer to multiply and move it about until you find a nice place for it, and go ahead and merge it with the continent (otherwise your layers will add up to epic levels in a short amount of time). Repeat over and over and over again until you have a bangin’ black and white map (see step5). Sometimes it’s a good idea to Gaussian blur your map by 1 pixel and threshold it again to get rid of awkward little fuzz around the coast.
Now, we begin the forewarned optional step. I use this step to make geologically realistic river systems, though, as previously stated, this is an optional step that can be skipped if you are confident of your river-making skills. This step is nearly exclusively borrowed from this tutorial: http://www.jezelf.co.uk/tutorials_map05.htm.
First, duplicate your map and invert the colors. Now the land is white (see step6).
Duplicate that and run a poster edges filter with the settings as high as they will go on the top layer. Duplicate that and run an accented edges filter on the top layer (see step7).
Gaussian blur that layer at about 7.5 pixels and put the opacity at 65%. On the layer under that, Gaussian blur it at 5 pixels, and drop the opacity to 50% (see step.