Power on your printer and manually raise the print plate via the menu. For my Photon S, this is done by navigating to Tools > Move Z.
Raise the print plate above the resin vat.
Loosen the two resin vat thumb screws, slide the vat out, and set it aside.
The print plate is mounted to a ball head that allows its angle to be adjusted. Use the included hex key to slightly loosen the bolt on the top of the print head.
Place a piece of paper on the print LCD screen and carefully lower the print plate until there is slight friction when pulling the paper. Adjust the print plate in increments of 0.1mm to carefully sandwich the paper until there's just a small amount of friction.
If you loosened the print plate to set its angle, make sure there's even spacing on all four corners of the plate, square the plate to the front or back of the machine, and then tighten the bolt to store the angle.
When you're finished, navigate to Tools > Z=0 to store the new Z-origin.
Use the menu system to raise the print plate and reinstall the resin vat.
You're all finished! Using this Photon bed leveling method, you shouldn't need to relevel for several prints.