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.
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.
The Anycubic Mega X is new enough that the machine doesn't appear in Cura's "Add a printer" interface.