Cutting or chopping a mortise is not that complicated if you practice and follow the following procedures.
Lay out the mortise, and clamp the work piece firmly to your workbench, because it is important that it the work is not allowed to slip while you are using a mallet and chisel. Always mortise to the exact width of the mortise chisel itself--adjust the dimensions of the joint to match the chisels you have.
Hold the chisel upright in your left hand (assuming that you are right-handed), but relax your grip slightly for comfort. Standing behind or just to one side of the work piece, hold the mallet in your right hand and deliver a firm blow to the chisel handle. Do not use a hammer. Always keep the chisel vertical, at 90 degrees. Make sure you leave a small amount of waste at both ends of the mortise for cleaning up later by paring those ends. Keep vertical to the surface of the wood (test with a square if necessary).
After the first blow move the chisel back slightly and strike the handle. Keep the beveled edge away from you. Wrap your fingers around the chisel handle, with knuckles facing away from you. Hold chisel firmly. Any waste from the second blow is left in the hole produced by the first strike. Remove this as you go by pulling back on the chisel handle. Start from the center of the mortise, working in one direction toward you. After a few strokes, turn the chisel around completely so that the bevel is facing you and remove the other half of the mortise while working away from you.
