How do i take the finish off of my guitar with sandpaper?

I currently own a Douglas Draco BK in black, and i want to spray paint the back of the guitar white. How do i go about doing this? I have purchased white spray paint, 150 grit sandpaper, and some clear coat. Do i just sand the finish down the back of the guitar, repaint it, and then clear coat it? Do i sand only the finish off the back? Or do i have to sand the paint off as welll? PLEASE HELP D: