Assuming that all the wall surfaces are ready
You need to start by marking horizontal and vertical lines on each wall or wall section which will act as a guide when fixing the wall tiles. The best place for the horizontal starting line is just above the sink and toilet cistern so they do not break the continuous line around the room. But it also important that this line does not then leave a small section of tile above the bath - this will depend very much on the size of the tiles you have chosen.
A good way to get the right height for this line is to divide the height of the bathroom in half, then use that line to mark the position of each tile below the line until you reach the bath. If less than half a tile height is left just above the bath then adjust the starting line up or down until either exactly half a tile or a whole tile will be positioned above the bath edge - don't forget to take into account the width of the grouting. Once you are happy with the line for the bath you then have to check it for all the other major elements of your bathroom such as the window, sink, toilet etc. to make sure no narrow strips are left in obvious areas. If there is no option but to have some narrow strips somewhere (which is often the case) then try to arrange the layout so that they appear above doors or windows or near the floor i.e. away from normal sightlines. With a bit of trial and error you should be able to establish a good horizontal guide line - now mark it clearly on the wall using a spirit level to make sure it is perfectly straight.
Next , you need to establish your vertical guide line in a similar way. Start by marking a line half-way along the longest wall and, again using your tile width plus the grout joint width, check all corners and edges of your sanitary ware to make sure there are no narrow strips of tile in the most obvious locations. Most rooms and corners are not entirely square so starting tiling from the middle of the wall will create a balanced layout. If you start in one corner and it is not perfectly square then by the time you reach the next corner you risk the tiles looking skewed as the tiles at the top of the room could be wider than at the bottom, or vice versa.
Finally mark vertical center point lines on all the other walls in the same way and with your wall surfaces well prepared and your layout guide lines marked you are finally ready to start spreading the adhesive and fixing the wall tiles of your choice. Look out for the next article in this series on how to install your chosen tiles...