I wrapped card stock around the tube for an edge to cut against.
Use a sharp knife and cut carefully. The paint will chip off if cut to quickly.
Use a sanding block with 400 grit sandpaper to clean and square up the edge. The coupler is already inserted for support of the edge when sanding.
Remember the coupler from Parts 6 and 7?
Join the two cut halves together.
Turn the upper tube 90 degrees - Voila!
If you took the extra time to make sure the tube was masked evenly, you should have a clean separation with the corners matching up. Nobody will notice the cut tube, the sharp color breaks will hide the line.