Step 5: Create Shelves/Tenons
1. Repeat steps 3.1-3 from creating the bookcase sides. In my humble opinion (as my priest says), I suggest you do so in the order I suggest (cutting the slots before the tenons) because you will have more stability at this point unlike if you were to cut the tenon then try to cut the holes. It just makes sense. These will not be slots, however. They will become your peg holes.
2. Cut your tenons. Next you will create your tenons, the second part of the mortise/tenon method. The tenons will fit through the slots on your bookcase sides. The most important thing to remember here is to cut inside the lines, as explained in the last step. Cut an L-shape out of the sides of your shelf ends. First from the sides, then from the top.
3. Repeat 15 more times. You're gonna be at this for a while. Might as well have a drink handy.