Step 5 · Connect P-Trap to Wall Drain
Align the trap arm with the drainpipe opening in the wall (the stub-out). In most cases, the trap arm will be too long and require cutting. To mark it, slide the arm into the wall pipe until it bottoms out, then pull it back slightly and make a mark on the arm where it exits the P-trap's J-bend connection. Or, more simply, hold the trap arm next to the stub-out and mark it for a 1 to 1.5-inch insertion depth. Use a hacksaw or a dedicated tubing cutter to make a straight, clean cut at your mark. It is crucial to cut straight. Use a utility knife or a piece of sandpaper to remove any plastic burrs from the inside and outside of the cut edge. Slide the decorative escutcheon (if provided) over the trap arm first, followed by the slip nut and the reducing washer (the washer is needed to adapt the 1 1/4" trap arm to the 1 1/2" wall pipe). Insert the cut end of the trap arm into the wall pipe, align all P-trap components so there is no strain on the joints, and hand-tighten all slip nuts.