If the Mortise is the hole that the Tenon fits into, I make them using a fence and a laser sight from a pistol which I hung from a rafter. I shine the light beam directly on the tip of the bit and mark the piece on the top. The laser light shines on the top of the piece and shows me where to start and where to stop. By hiting the marks I have drawn I follow the light to the next mark knowing that the bit is directly below the light and is doing its job of making an exact cut. I use the same method for making initials on the head board of doll beds. I use a computer to make the initials, copy them into the Paint program and ask that program to reverse them. I print that and tape it to the top of the piece of wood. The laser follows those letters and makes a correct copy on the reverse side.