A drill press with a mortising attachment is the easiest and most efficient way of cutting mortises for mortise-and-tenon joints. The mortising attachment consists of a rotating bit inside a square chisel; the bit cuts a round hole and then the chisel carves out the corners. The key to using the mortising drill bit successfully is to make sure that you have set the chisel square to the fence so that your mortises will come out perfectly centered.
Outline the mortise on your workpiece. Make sure that the outline is centered between the edges. Put the workpiece on the drill press and lower the chisel to check its alignment with the workpiece outline. Adjust the fence as necessary so the chisel lines up with the outline. Set the drill depth if you won't be drilling completely through the workpiece.
Set the drill speed in accordance with the type of wood and the size of the chisel. Slower speeds are appropriate for hardwoods and large chisels, while faster speeds work better with softwoods and smaller chisels. For example, if you are drilling a quarter-inch-wide mortise in hardwood, set the drill to 1,200 RPM; if drilling in softwood, the drill can be set to 1,500 RPM.
Place a scrap piece the same size as your workpiece on the drill press and make some practice cuts. If there is a lot of smoke coming from the wood, lower the drill speed. Check the placement of the cuts and make any necessary adjustments.
Put your workpiece on the press and lower the hold-down arm, making sure the piece can still move freely along the fence. Make your first cuts on each end of the mortise, then stagger the cuts in between until the mortise is complete. After you give the bit enough time to drill the hole, you'll need to pull down a little to get the chisel to carve out the corners. Stop every so often to clear away the wood chips.