We'll now turn our attention to the mortise and tenon joints that will hold the frame together. You'll need to cut a mortise in each stile for each of the tenons (to be cut on the ends of the rails in the next step of these free woodworking plans).
TIP: Remember, when forming mortise and tenon joints, it is always easier to cut the mortises first then fit the tenons to the mortises rather than the other way around.
Probably the easiest way to form mortises is with a dedicated mortiser, which is essentially a drill press with a square chisel surrounding a drill bit. The drill bit in the center removes most of the stock, while the square chisel does the rest. This allows the operator to drill square holes, perfect for a mortise. Some drill press manufacturers offer optional mortiser attachments with various sizes of square chisel bits.
However, if you don't have a mortiser, there are a number of other ways to create an appropriate mortise. You could mark out the mortises and remove most of the material with a drill press or power drill, then clean up the mortise with a sharp chisel.
You could also use a straight bit on your router table in place of the drill bit, by easing the stock down onto the bit, moving along the fence and then raising the stock back off of the bit. To make a wider mortise, simply move the fence a bit and take another pass. Then, clean up the mortise with a sharp chisel.
Your mortises should be about half the width of the stock. In our case, the stock is 1-1/2" wide, so we made our mortises 3/4" thick, centered in the width of the stile. And since we won't want to see the edge of the tenon in the bottom of the frame, we'll cut the mortise back 1/2" from each end. On our 2-11/16" wide rails, we made a 1-11/16" wide tenon, so we were sure to start the mortise back 1/2" from the edge.