I took off the access panel to see how I made the install. It is much easier for two people to complete the mod. I did it alone but I'd recommend a second set of hands.
Working from memory I put together the following steps. I also posted a few more pictures to help explain the install.
1. Remove existing fixture and cut water lines (cut off as little line as possible at this point). 2. Enlarge the faucet holes in the bathroom wall to accommodate both the shark fittings AND the black plastic nuts (note: the holes will be quite large which is why I added the PVC back panel and the shims).
3. Cut the PVC backer to size and drill holes in the backer for the hot and cold water faucet lines. 4. Slide the backer on to the faucet. 5. Screw on the plastic nuts just far enough the catch the threads. 6. Screw on the Shark Bite fittings. 7. Apply caulk to the back of the PVC. 7. Insert the fittings, plastic nuts, and faucet lines through the holes and seat the PVC panel by pressing it against the shower wall. You may have to insert the assembly at an angle to get all the pieces through the holes.
8. From the back side tighten the black plastic nuts to hold the faucet and panel in place. I had to add shims because the holes were too large for the plastic nuts to grab. I suggest letting the caulk set overnight before attempting to attach the blue and red lines
9. Cut the blue and red water lines to length and insert in to the shark bite fitting.
I discovered that the shark bite fitting sits about 3/8" proud from the back panel. This required that I add a 3/8" strip of wood at the top and sides of the access panel. Doing this gave the needed clearance. Below are pictures of the back pane and my mods.
I hope that this helps - don't hesitate to reach out if you have additional questions.