Whenever you decorate near or around an electrical fitting, always turn off the mains power at the consumer unit, and identify and remove the fuse supplying power to that circuit. It is not enough just to turn off the power at the switch. Wallpaper paste, paint and tiling adhesive are all wet and can make a lethal combination if they come into contact with electricity.
Once the power is safely off, tiling around switches simply means measuring and marking tiles that need to be cut to size. Use the method that suits you best – hold the tile over its neighbour and mark the cutting lines, or make a cardboard template.