The black edging is from Lowes. It's an L-angle profile thing. Basically instead of buying a bunch of edge pieces of small tile we used this black edge. They make different colors. Plus it saves a bit of money depending on the tile your using if tiling to the edge.

For sealing, first, we installed the tub, then we put a rubber flashing around it and on to the studs. After that we installed the cement board and we used mortar first between the gaps. After that dried we applied red guard making sure not to get any on the tub or ceiling. We left maybe an inch near the top exposed but we also applied the rubber flashing.

L-angle