I know it is a simple procedure but here is how I painted my trunk.
Clean out the gutter, bottom of rear quarter & trunk. Make sure drain holes are clear. 2 on each quarter for my Nova.
Vacuum & wash everything.
Wire brush & sand any dings or rust. Don't go crazy as the spatter paint hides a lot.
Wipe everything down with Wax & Grease remover.
Mask off everything not to be painted,including under the trunk lid. I remove the side marker lights and mask off the tail light wires.
Start with a base coat of satin black. This saves using a lot of splatter paint. With my Nova 1 can of black & aqua spatter paint was barely enough. Paint with swift strokes to start. It doesn't take much paint for the desired effect. If you have any paint left touch up where you think it looks less spattered

. Don't go crazy. Too much Aqua does not look stock. If you don't like the way it turned out you can paint over with the satin and try again.