What are you using for color? Spray paint or something? Or is there a special dye? Does BMW have dye codes for various leather seats? I'm assuming you have leather seats.
So you deglaze the current leather to "rough" up the finish. Apply the dye by cloth? Then what, buff it somehow? Don't need a detailed DIY, but just a general overview. I have a friend with an E36 who has a black scuff on his tan leather seats and the dealer wants $$$$ to fix it. (I'd only charge half the $$$$ ;^)