3. Cut the Mortise
Use a chisel that is equal to the width of the mortise, if possible. Cut the top and bottom of the mortise first. (Hold the chisel perpendicular to the surface with the tip on the line scored by the butt marker and the beveled side facing toward the mortise. Strike firmly.) Then cut a V-channel across the mortise anywhere within the mortise. (Hold the chisel at a 45-degree angle with the bevel facing downward, making two cuts at opposite angles.) Next make a series of cuts, spaced about 1/8-in. apart, starting at the V-channel and working toward the outside edge of the mortise. Finally, use the chisel to scrape out the chips; smooth the bottom of the mortise; and clean up the perimeter with perpendicular strokes.