The cutting is pretty standard except for the sides of the shelf lip. I did two different ways mostly b/c I forgot when I was cutting what I had decided to do. It works out either way, just with a little different look. Option 1 (picture 7): tapered top as it meets the shelf above it--measure the distance from the bottom of the shelf to the bottom of the next shelf. Use that measurement to draw a diagonal line from corner to corner of that measurement. Then cut the board at that length. Cut on the diagonal line (the hypotenuse of the triangle will be the cut line and will go against the vertical side.) I found it easier to adjust my miter angle so that the blade matched the line I needed to cut and yet the board was still flush with the back plate. I needed to turn the board and line it up again to cut through the board (since it was longer than 12"). See the pictures for a better understanding. Option 2 (picture 8): chunky top with the tip trimmed to fit under the next shelf side. Measure the distance from the bottom of the shelf to the bottom of the next shelf and cut the board at that measurement. Then cut the board diagonally. Again, I found it easier to adjust my miter angle so that the blade matched the line I needed to cut and yet the board was still flush with the back plate. I still needed to turn the board and line it up again to cut through the board (since it was longer than 12"). Once cut, you can use the next shelf side to mark the angle to trim off (I used a hack saw since it was late, but you could use the chop saw). See the pictures for a better understanding.