aj ball

centre the window check the reveals are straight if not cut the tiles all the same from the furtherest point so when you put them up you create a straight reveal because if you don't and cut on the pee when you put your trim on it will pick it up and it will look awful then back fill so the inside of the reveal becomes level with the tile best to leave over night to set depending on how much filling needs to be done it work's everytime can be a bit of a pain to do but it show's in the finish let me know how you get on good luck