Marking the Slots
Place the door on its edge with the hinge side facing up. Measure the centers of all three hinges on the old door with a tape measure, taking the measurement from the top of the door down to the bottom. Transfer the measurement to the slab door and mark the centers with a pencil.
Adjust a combination square so that a quarter inch of the blade extends beyond the straight edge. Place the straight edge of the combination square against the front face of the door with the edge of the blade over the door edge.
Place a pencil on the end of the blade and blade the combination square across the door at the hinge center marks. This gives you a guide for the back edge of the butt hinge leaf.
Place a hinge leaf on the door edge, lining up the center of the leaf with your center mark. The edge of the leaf opposite the sleeve should line up with the trace mark you made with the combination square.
Trace the outline of the leaf onto the door edge. Hold the leaf inside the tracing with half of the leaf over the edge of the door. Place another hinge leaf under the overhanging half and trace the bottom of the second leaf onto the back face of the door. This gives you a depth mark. Repeat the process for the remaining hinges.
Cutting the Slots
Hold the blade of a 2-inch wood chisel at one end of a hinge slot tracing. The straight edge of the chisel should be against the line with the beveled edge pointing toward the center of the slot. Tap the chisel handle with a hammer until it penetrates the wood down to the depth mark.
Pull the chisel out of the cut and reposition it a quarter inch closer to the center. Tap the handle until the chisel blade cuts down the the depth mark. Continue making new cuts a quarter inch apart until you reach the opposite end of the tracing.
Place the flat edge of the chisel against the bottom of the depth mark. Tape the handle of the chisel with the hammer, removing the wood from the slot. Move down the hinge slot until you remove all the wood from the hinge slot.
Sand the slot with a medium grit sandpaper to level the slot. Test the fit by placing the hinge leaf into the slot. The top of the leaf should be flush with the door with the hinge sleeve against the door edge.