ok not exactly a perfect guide but here it goes:
1. Certify the washroom is empty, as you don't want to be caught
2. Position yourself silently at the stall next to the one you're trying to peep.
3. Hold your phone way up high so you can first see the top of the head and figure which way the guy is facing, then proceed to film it from his back slowly trying to steal dick shots when he lowers his head under the shower.
4 Don't choose short guys, as their head tops are lower and when you can see it with your phone they can probably see your phone back.
5. Don't film minors, file your photos and videos constantly and try to keep the lesser you cant off-cloud.