A mortise is the term used to describe the hole or slot cut into a piece of wood that accepts the end of another piece, called the tenon. Mortises are typically rectangular in shape and have squared ends to accommodate the square ends of the tenon. This helps prevent the joint from twisting. There are many ways to make mortises, and many jogs and machines have been created solely for making mortises. If what you need is a perfectly square mortise, you can skip all the fancy machines and use just a few basic tools and techniques.
Measure and mark the location of the mortise with the ruler and a pencil. Lay the ruler over the area you just marked and draw a line between the opposite corners to form an "X" inside the square. This is your center mark. Now, trace the layout lines with your knife and ruler. Scoring with the knife helps to keep you from going past the lines when making the mortise.
Select a forstner bit that has a diameter that's slightly less than the inside dimension of the square mortise. Place the point of the bit in the center of the square where the lines intersect Press firmly or tap the end of the bit with a hammer. This gives the point of the bit a place to go and ensures that the bit will hit the exact center when in the drill press.
Chuck the forstner bit into the drill press. Set the depth stop on the drill press to go 1/16 of an inch deeper than the final depth of the mortise. Lay the workpiece on the drill press table and secure it, making sure the bit will hit the center area you punched earlier.
Start the drill press and lower the bit at a smooth and even speed. Drill down partway then raise the bit to allow chips to escape. Do this a couple of times as you drill to prevent the bit from clogging and burning the wood. Once the mortise is drilled, stop the drill press and move the workpiece to your bench.
Select a sharp chisel the same size as or a fraction smaller than the square you laid out. Holding the chisel firmly with two hands, pare the corners away to make the round hole square to the lines you scored with a knife. Take very light slices and keep the chisel straight and parallel to your work. Pare away each side until you reach the knife lines.