So the walls are painted, the new electrical fittings are in place. Nothing left for it but to do the tiling. Tiling is a perfect lesson in error accumulation in 2-D geometry, and you have to be alert to possible traps, every step of the way.
First, decide which wall you want the tiles to be parallel to (rooms are never square) then draw a line in the middle of the room parallel to that wall, with a pencil. Decide how far from the parallel wall your line of tiles is going to be (at the edge to which they are parallel, you want to have to cut the tiles to a little under half their width), and mark out this line, parallel to your first. This is your tile line. Decide how you are going to position your tiles along this line, according to whatever aesthetic principle you like, and mark it.
Position a piece of taut string above your tile line, since when you put tile cement down, your line on the floor will be hidden. Now you can start to cement the tiles. Use a straight edge at all times to make sure your line of tiles is straight, and spirit levels to make sure they are level, and eyeball them to make sure they are the same height. When you are happy, let your line of tiles dry, and tomorrow they will be the reference point for the rest of your tiling.
Day 1: straight line. Day 2: two more rows of tiles each side of the straight line.
Meanwhile, Spring is happening in my garden, completely without my help.