It probably wasn't the safest method - so I can't recommend doing it unless you're experienced or comfortable using a table saw.
Basically I cut it to rough length, leaving it a half inch too long on purpose. Then I dry fit the end cap. You can see the end cap has marks/bars all the way around it - I used those as my guides for making cuts. I marked 1 of those bars with a sharpie and put a corresponding mark on a stop block that I clamped to the fence behind the blade. That was my start/stop reference when the black marks were aligned in the 12 o'clock position.
Then I set my fence in place so that the blade was exactly in the middle of the pipe (with end cap on). I raised the blade just enough to get the length of intake slit I wanted. Then I put a second end cap on the other end so I could keep the length of PVC straight & parallel with the fence.
Then I'd slowly lower the pvc onto the blade and raise it straight back up. Using the stop block table interface as a pivot point. I'd raise the end if the pipe I'm holding back up to say 45 degrees, spin it so the next bar on the cap is in line with the black mark and lower the pvc back down onto the blade. Repeat until the initial mark on the pvc end cap has gone all the way around and is back at either the 11 or 1 o'clock position, depending on which way you spin the pvc.
Then I cut to final length and used a thin file to clean everything up.
I have to repeat: this was not the safest way to use a table saw so don't do it if you're not comfortable using one. The pvc was long enough that I was never in danger of losing fingers, but the potential for 'kick back' is high.