Start in a corner, preferably near a door. Use a plumb line on every sheet as you hang it ( you can put pencil lines on the walls to make it easier to line each one up) At every corner, instead of trying to paper round it, cut the last pece of paper so that it goes 1 - 2 cm round the corner (each piece will need to be wider at either the top or bottom, where the walls aren't true) The start the next wall exactly to a plumb line again so that, if necessary, the corners overlap. It takes a bit of practice and patience so avoid big prints, stipes, etc. Just make sure that the pattern runs true round the top of the room!