This page contains a bunch of helper images which one can use to get editing image done quicker.
Freecraft requires a dithered shadow, since it not that easy to create with Gimp, here is a finished black dither shadow. Just copy it to a new layer, remove the parts you don't need and you are done with shadowing your building/unit.
Getting the perspective right, can require quite a bit of work. To make it easier I have a blender file which I can use to generate a prototype wireframe rendering of buildings which I paint then in Gimp.Blender file