Measure the access panel's width and height. Your tile's grout lines must line up with the sides of the access panel so you can get the panel off without disturbing the tiles.
Plan the tile installation for the tub deck so the grout lines match up evenly.
Cover the perimeter edge of the access panel with a strip of painter's tape. This prevents the mortar from seeping into the perimeter gap and fusing the panel to the tub deck.
Begin tiling the deck and access panel together once you are certain that the access panel is installed in the tub deck securely.
Spread the thinset mortar onto the deck from the bottom up. Smooth the thinset onto the deck using the flat edge of the trowel, then comb the mortar with the notches until it is a uniform thickness.
Press the tiles into the mortar firmly and lay a beating block on top. Hit the block firmly with a rubber mallet to drive the tiles evenly into the mortar.
Remove the access panel and peel off the exposed painter's tape once the mortar has cured for 24 hours. Some of the tape may be under the tile.
This is fine; it will not affect the final installation.
Replace the access panel and grout the tiles. Spread the grout onto the deck with a grout float, pushing the grout into the joints from multiple directions.
Grout the interior of the access panel at this time, but do not grout the perimeter.
Let the grout set up for 10 minutes, then clean off the tub deck with a damp grout sponge. Work the sponge in circles over the deck until the tiles are clean.
Run a bead of latex caulk into the gap around the access panel. When you need to remove the panel, cut the caulk with a utility knife to free the panel.
Allow both the caulk and the grout to set up for 24 hours.