Make a guaging rod to start with by laying tiles with spacers in alongside a long straight piece of timber and mark the joints on the rod, vertically on one side and horizontally on the other. Next mark your centre lines horizontally and vertically on the window wall, use the gauging rod to find out the best layout for the tiles, repeat this for the other 3 walls but if your going brick bond make sure the corner cuts look like the tile carries on from one wall to the other. the gauging rod can also be used to find out where the tiles will fall in relation to other obstacles on the wall. never start with a full tile from a floor, bath, shower tray etc instead mark where your first full tile will be at the bottom of the wall and place battens on the wall here but make sure they are perfectly straight or your grout lines will not meet up in the corners. once marked out prime the walls, make sure you put your adhesive on nice and evenly and apply tiles with even pressure and they should be nice and flat with no lips. hope that helps, im sure others will chip in with more tips