Many do-it-yourself tile installers go at the job with no holds barred, until it gets to the part of tiling around windows. This is a tricky area that takes some thought before you just simply apply the thinset, stick on the tiles, and then let it go.
You could come back later (or the next day) and find some or all of the tiles you have so carefully set on the wall surrounding a window either well on the way to sliding off or off the wall completely, maybe even lying in pieces in the tub or on the floor.
Lay a Wood Strip for Support
Tiling around windows to avoid the above disaster simply takes some planning and forethought. It is important to lay out your tiles beforehand on the floor or other flat, unobstructed surface, so you can ensure the design and how the tiles will look.
Then, the trick lies in running a course of tiles FIRST from the top part of the window. This leaves a blank space at the sides of the window opening. For this course to stay stuck to the wall long enough for the adhesive to set it so that it stays on by itself, tack on a strip of wood at the bottom of where the tiles will rest BEFORE you attach the tiles. Since the sides are still blank (without tiles), you can attach the strip of wood to the backer board.
The strip of wood gives the tiles something to rest on while the adhesive is drying and prevents them from slipping downward and off the wall when they reach the window opening.
After this course has dried (give it a good 24-36 hours), you can easily take off the strip of wood and apply the rest of the tiles above this course without fear of them falling off since they will be resting atop the spacers you have put between them and the first course you have installed previously.
Angle the Corners
Another tip for tiling around windows is to make angled cuts at the corners, especially if you are using trim tile (bullnose and liner tiles). In so doing, you will be able to keep the pattern of the tile nice and neat and flowing from one section of the window to the next. It will take some measuring and figuring, but in the long run, you will be much happier with the results.
Simply cut the corners at 45-degree angles and after setting them with thinset, place a strip of masking tape over them to hold them securely in place until dried.
The biggest mistake people usually make is to cut the corner going the wrong way, so be sure and think before you cut! Marking the tiles in the direction they should go with a pencil or marker on the backside helps to keep you cutting in the correct direction.
Tiling around windows is not hard; it just takes a little more time than the rest of the job. So be patient, use your head, and you are more likely to come out with the results you want.