I used Photoshop, but it can be done with gimp as well. This is actually my first normal map, but I've done few bump maps before. Last step before you create a normal map, is to create a bump map (for simple tile textures). Bump map is gray scale image, where higher things(tile) end up whiter, than ...

icaro440 no offence, but that is one of the worst normal maps i've seen. Create new one or use mine.

on left is your normal map, on right is mine