Step 3: Cutting mortises and tenons
This is the fussy step. I started by cutting a prototype tenon, about 1/2 inch deep, and slightly shorter than the full width of the 1x4. Note that my tenon has round edges because I cut the mortise with a 3/4 inch router bit. You could use square edges if you use a chisel or dedicated mortising tool. Next, I had to layout 3 3/4 inch holes spaced 120 degrees apart (if you're clever you can do this with a compass). So I started with top shelf, and marked the hole centers about 1.5 inches from the edge. I stacked the top shelf disk on top of the bottom shelf and centered them, and then carefully drilled 1/16 inch holes through the top into the bottom to transfer the layout marks. I marked the holes A, B, and C on both disks. I then used these holes to center a 3/4 inch spade bit, which I used to drill some starter holes about 1/4 inch deep. Remember that the mortises go on the top of the bottom shelf, but on the bottom of the top shelf. You want to use A, B, and C markings to keep track of the corresponding holes for the top and bottom shelves. That way, if the initial spacing was slightly off, you aren't compounding the error by changing the orientation of the top and bottom shelves.