I've only tried to do this twice in Photoshop - it's not the correct program for doing such things but it's all I have so here's as far as I got with it.

First make your continents white on a black sea...very basic cow spots type thing. You'll want your coastlines as detailed as possible so it should be the "final shape". Next duplicate the layer. Use the reactangular marquee tool and draw out a selection that covers the top half of the image. Hit Edit-transform-scale and reduce the height 25-50%. Deselect and move to the top of the screen. Select-color range=black, hit delete and deselect. Now your continents have a distorted look but they don't line up properly anymore. So here you'll have to manually stitch them back together with the pencil and eraser. Then you do the whole thing for the bottom half of the screen. Once you have these basic shapes they will serve as the templates for doing the "pretty" work of adding color and terrain.

Again, this isn't perfect but it's as far as I ever went with it...not sure how to do it in other programs either.