Tile gauge or tile rod
Make a gauge rod using an old offcut of 2" x 1" wood or similar, which should be reasonably straight.
Lay out several tiles on the floor, spaced as they will be when fixed. Either use plastic spacers, matchsticks or the lugs on the tiles themselves to allow for the grout between each tile.
Place the rod next to these and carefully mark the line of each join.
You now have a suitable rod for marking out tile positions.
Setting out plain Walls
Measure the height of the wall and divide by two.
Mark this height on the wall.
Using the gauge, lightly mark tile joins from this line down to the bottom.
If the size of the last tile will be less than half a tile, move the original starting point up or down by half a tile and repeat the process. If the size of the last tile is greater than half a tile, keep these marks.
Horizontal levels and tile battens
Using a spirit level or water level, draw a horizontal line across the wall for the bottom of the lowest row of whole tiles.
Secure a batten to the wall along this horizontal line to serve as a base from which to tile.
Vertical tile guides
Now measure the width of the wall, mark the centre and repeat the laying out process horizontally.
Again, if you are left with less than half a tile at the ends, move the original mark by half a tile in either direction.
Mark the tile joins from your gauge rod and, using a spirit level, mark a vertical line for the middle tile.