How to properly seal window in tiled shower stall.

Proby

I will be turning my bathtub into a stall shower in a few months, but I am reading up on it now.

I have a window in the shower, currently that is one of the problems, water leaks down. The person who re-did the bathroom 3 years ago before I bought the place used sheetrock and mastic so everything needs to be gutted.

The window is a vinyl replacement, it is set back about 4" from the shower wall (it's a brick building and wood framed, all the windows are like this). They put 4" tile down as a window sill and it was leaking bad until I put plastic across it.

I am going to have a professional tile guy do the tile, but I would like to set the shower receptor and put up the backboard myself, along with any sealing that needs to be done.

What would the proper way to seal around this window be?

FWIW, when I get started and do the demo, I will take pictures. But for now, I am just curious.