The trick is remembering the corners of the room, aren't square / true, no matter how new the house is. So you need to fit the first piece, to a plumb line (true vertical) less than one drop width from the first corner. So you can cut the second piece into the corner. Also you need to plan so that you don't end up with a narrow strip in the other corner.
Final tip, you need to make sure you have enough paper, to be sure you can match the pattern repeat (if it's not a plain colour).
Got to fess up, haven't wallpapered anything for a very long time. Got to be at least 30 years.
Funny story. Back in the mid 90's we bought a new house. Had loads of paper to decorate, but only ever put borders on painted walls. Our next door neighbours also in a brand new house decided to paper, and started with said plumb line. Tapped in a pin, straight into the main house gas supply that was way too close to the surface of the wall......... Sad to say, the quality of new house building has got worse, not better!

