Order matters, as it does on a real job: a slip nut and its washer have to be on the pipe before that pipe goes into its fitting, and the nut always goes on before the washer, which sits at the pipe end. At the wall it's the other way round: the washer goes into the adapter's hub and the nut threads on over it, then the arm slides through both. Every washer's taper points into the fitting. Tighten slip nuts by hand, then a quarter turn more.

    White: 1-1/2″ tubular drain partsBlack: tapered slip-joint washersBlue: water during the testGreen ring: a spot the part can go now (red: not yet)A 1-1/2″ tubular P-trap: 12″ tailpiece, J-bend, trap arm into a wall trap adapter, three slip joints. Its water seal is 3.64″ deep, inside the 2″–4″ that model plumbing codes ask for, and the arm falls about 1/4″ per foot to the wall. Slip joints only belong on the trap inlet, the trap outlet and inside the trap seal, and they have to stay reachable. Check your local code and the maker's instructions.