10. Plane the inner edge of main frame

Lay out your boards.
Mark the inside edge.
Take a planer to the inside edge of the frame on all four boards.  You can use an electric planer like above, or a manual planer (which is safer). 

By planing the edge, you make sure that a harsh and unwanted line, caused by the wood's inner edge touching the canvas, doesn't show up on your artwork once  after you begin painting.