If the wall tiles need to line up with the floor start with the floor by setting out the floor you'll have to just line the walls up from there.
If floor tiles are different size to walls then start with walls check your heights of fixtures (Taps sills Bath etc) first and work out what cut you'll have at cornice and floor leave bottom tile off to cut in after floor is laid. Always try and centre the window (Not always possible) next work out widths aross the room so you have good size cuts.
So what you are really trying to do when you first get there is.
1 check tiles quanity and size
2 check and double check lay-out (Freize tiles feature walls ec) with client and make sure you and the client fully understand each other and agree on what needs to be done
3 set-out tiles horozontally then vertically for best cuts and looks
4 decide on whether the floor is tiled first or the walls depending on tiles sizes and layout.
5 take your time get it right before you start fixing anything
6 don't be nervous sell yourself.
7 take the camera and if you have a problem stop take some pics and post them and we can give advice.