When tiling any surface, you should always lay the whole tiles (field tiles) first. Once all of the field tiles are laid, you need to go back and cut the tiles to fit around the outside (border tiles) and around obstructions such as window frames and light fittings.
Cutting straight edged border tiles is relatively easy, but make sure you measure the gap for each tile rather than just measuring one gap and cutting all tiles to that width. Walls, especially in older houses, are very rarely perfectly straight.
To measure exactly where you need to cut a border tile, hold it face down against the tile it will be next to and with its edge against the adjacent wall or edge of the tiled area. Mark the back of the tile where it overlaps its neighbour (allowing 3mm for normal spacing) and then transfer the marks to the edge of the tile. You can now score the face of the tile with a tile cutter to penetrate the glaze and snap the tile along the score mark. To get a nice clean break, pin a length of thin wire to a bit of scrap hardboard. Lay the tile on the wire so the scored line is over the wire and then press gently down on both sides of the tile.
Clean up the cut edge of the tile with a tile sander or slipstone. You can now lay the tile into position. Make sure you don't slide it into position or you will move the adhesive bed and weaken the fixing.
Safety!
Always wear goggles when cutting tiles to avoid small chips getting in your eyes.
Make sure the room you are tiling is well ventilated. Tile adhesive and grout can be noxious and breathing too much of it is not a good idea.
Click here to read more from DIY Extra