It's pretty simple in my opinion.
When creating your textures, think about which parts would be raised up above the surface of the object in question were it in the real world. Once you have the texture saved, make a second grayscale texture with white being the raised parts and black being the crevices and stuff. You then import it to unity, set it to convert the asset to a normal map (make sure to check the create from greyscale box), and have the texture remain the same as the source file (32x32 is the smallest size unity allows).
Be sure also to resist the urge to bake any lighting effects at all into the regular texture. That's what the normal map is for. Your regular texture should only encode albedo.
