An installer will remove the old tile, inspect the substrate, and repair or replace it as needed. For showers, they should apply a waterproof membrane over cement board or use a foam board system, then set tile with thinset suited to the tile type. On floors, they'll check for deflection and add an uncoupling membrane where the floor needs it.
Tile alone may not need a permit, but replacing a shower pan, moving a drain, or adding a heated floor usually does. In Wareham, permits are filed online with Inspectional Services, plumbing inspections are held Tuesday through Thursday, and the posted permit card must be at the house for each visit.
Expect curing time between steps. Thinset, grout, and some membranes need hours or days before the next layer or before the shower can be used, so ask for a written timeline.