Find the location of the mortise on a piece of 2 x 4. Make the marks with an edge guide or use a jig to hold the sides of the 2 x 4. The jig limits the movement of the router during the cut. Mortise-and-tenon joints are usually cut on a table saw, but they can be made using a router as well. The joints are rounded when using a router, but they are just as strong as squared-off mortises and tenons. Cut the mortise with a plunge router. Make one pass about halfway through the mortise, then plunge all the way on a second pass. Use the same principles for whatever wood is used in any project; just adjust the size of the router bit.
To cut the shoulders for the tenon, place a board flat on the work table and clamp it down. Run the router across the board. Turn the board over, clamp it down again, and cut the other shoulder. Sand again, if necessary. Round off the corners of the tenon with a utility knife.