Once you get it all nice and straight you may find you have a few different colors showing. If this is the case, you probably want to seal the area with sealer of a similar shade to your final color. This will do 2 things. First, it will give you an even ground coat so it will take less paint to achieve full coverage. Second, it will prevent the possibility of solvents from the base coat getting under an edge of the previous layers and wrinkling on you.

Buy 2' level from home Depot to check your body work.