Is there a repair guide for replacing the glass back of the 12 Pro?

There will likely be one in the future on iFixit, however it's not really an easy process at the moment so not practical, especially for the DIY repairers.
*EDIT: Just had a quick look and there is a guide that involves the laser machine here
I'm lucky enough to work with a $4000 laser machine that is used to laser off the adhesive between the glass and frame so it can be broken and chipped off slowly. We also have to use specific diagrams when lasering or else risk lasering an antenna from the device. Then the replacement glass is glued on and bonded down.
If you don’t have an expensive laser removing all of the old glass and adhesive is really tricky o do cleanly and if a small piece of glass is left there when the replacement glass is put on, it’ll all shatter and all be for nothing. It’s a pain because of the way Apple has decided to construct the device, but I’d recommend taking it in to a repair shop for this repair as they can maybe do it same day and with all the correct equipment, but without charging as much as Apple might.