First off all you need to measure the length of the down tube on the frame and along the top of the frame, then you need to work out the angle of the frame so that you get a even gap between the frame and the bottom of the shelf. Then mark and cut this out of the ply so that you have 2 pieces of ply these should be cut at least 200mm wide ( one for the top and one for the down tube )
Step 3: Cutting The Sides
Next I sanded them down and glued and pinned them together and then I could mark out the 2 sides by putting it on to a piece of ply and drawing around it then using a straight edge draw a line where the 2 pieces of ply end, that's your triangle template. Cut these out and sand them down, then glue and pin them.
Step 4: Paint
Next drill 2 holes in the back of the shelf and then paint with undercoat, finally paint with gloss and once it's dried mark the 2 holes on the wall and drill them plug them and screw it up!