After the mortises have been cleanly cut, we'll make the corresponding tenons. These tenons can be cut a number of ways: you could cut them by hand with a small hand saw, use a band saw or cut them on a table saw with a tenoning jig. In our case, we chose to cut them on a radial-arm saw using a stacked dado blade set. (You could accomplish the same task with a table saw using the miter gauge.)
Set the depth of cut to match the width of the amount of material you left behind when you cut the mortise - in our case, we will be cutting out 3/8" off of each wide edge to leave behind a 3/4" thick tenon. If you're using stock thicker than 1-1/2" then you'll need to adjust the depth of cut.
After cutting the two flat sides of each of your tenons, adjust for a deeper cut (1/2" in our case). Then turn the stock on its side and make the remaining cuts of the tenons.
TIP: Always cut your tenons a bit larger than their finished size to begin, then try to dry fit the tenons. If they don't fit, cut them a little further and make any adjustments that you deem appropriate to ensure a perfect fit.
After all of the mortise and tenon joints have been cut and dry-fitted properly, disassemble the joints and mark the corresponding joints in discreet locations so you know how to re-assemble the frame in the final assembly. Then, sand all of the exposed sides of the rails and stiles.