You need to use measurements and stick to them. So you have your straightedge on paralell line and then your square line. Say you are using 200 x 200 then take measurement of 5 + joints and use this to move away from the square line . If you want to and you are doind a largish room then create chalkline boxes by paralelling in each direction from the square line. Then just keep every 5 tiles inside that box and adjust the tile joints to fit . This comes in handy when you fit so called 'modular' tiles , in tilers speak they mean SIZEY :0).
This is how I used to do large quarry tile floors years ago ,in the days of 300m²/400m² tiled floors.
Hope that made sense.
Diggy