Dry-fit everything first. Put the trap adapter onto the metal tailpiece. Slide the PVC pipe into the other end. Look at the curve. If the pipe touches the wall, it’s too long. Mark it with a pencil. Use a hacksaw or PVC cutter to cut it. A hacksaw leaves burrs. Sand those down with sandpaper. Seriously. Burrs catch hair and cause clogs. Yes, your future self will thank you.
Fun fact: PVC cutting is oddly satisfying. It makes a clean snap sound with a proper cutter. It’s the ASMR of home improvement. Don’t fight the urge to cut it again just for fun. You’ll run out of pipe.
The "Oops" Factor
If you cut it too short, you can use a PVC coupling. It’s a little repair link. Or you can just buy another piece. PVC is dirt cheap. A four-foot section costs less than a fancy coffee. Mistakes are cheap here. That’s the beauty of plastic. You can’t say that about copper or brass. Those are the divas of plumbing.