Protect the preformed shower pan
A simple way to protect preformed shower pans during tile installation is to use the shower pan’s cardboard box. Mark the shower floor measurements onto a cardboard template and make any adjustments needed. Carefully indicate where the drain hole is, then trace this cardboard template onto the preformed shower pan. After installing the shower pan, use the cardboard as protection from shoes, dropped thinset, buckets, etc. Typically, when I tile a shower I use a ledger board one row up, followed by the shower floor and then the final bottom row. When it is time to tile the floor, the preformed shower pan is in great shape. When I come back to grout, I use the cardboard one more time to protect the floor from falling grout, drippy sponges, etc.
Using the box the preformed shower pan came in, cut a template to fit the floor area. This allows for revisions prior to tracing the template onto the actual pan.
Line up the drain hole then mark the outline of the template.
The cardboard protects the preformed pan during tile installation and, after trimming slightly in length and width, grouting and washing.
The floor is ready for grout with minimal clean up.