Tiling well is difficult. Tiling is where any imprecisions in the horizontals or verticals, or in angles that are supposed to be 90°, all come back to bite your bum. You can easily spot the mistakes, as the tiles that are supposed to be four-square, march in zi-zag pattern up the wall, or thin lines of grout widen to rivers along their length.
I don't claim to be expert in this. On the plus side, grouting can cover a multitude of sins, and with this in mind I used a brilliant white grout, the same colour as the tiles. Grouting in a contrasting colour is for the very brave or the supreme expert, neither of which description applies to me when it comes to tiling.
One of the things I discovered in the DIY shop, and which worked well, is a new kind of spacer. It's designed not only to space the tiles accurately in relation to each other, but to make sure they are all at the same height so that they make a flat surface. The manufacturer is Pavilift, and there is all the info you could possibly need on their website.
This photo shows them in action in the shower cubicle.. Tip: make sure there is no tile glue behind the black clamps, as it can scratch the tiles. Also, make sure that the tiles don't move as you tighten down the clamp.