Draw your own vertical, because the room has not got one
The instinct is to start in a corner, because a corner looks like a straight edge to work from. It is not. Corners in houses lean, bow and taper, and a first length hung against one is leaning before you have hung anything else.
Every length after it copies that lean, so by the time you have worked round the room the pattern is running uphill and there is nothing to be done about it. Draw a plumb line on the wall with a level or a bob, hang the first length to that, and let the corner be as wrong as it likes in a place where it does not read.
Then draw a fresh plumb line on every new wall. The corners do not pass squareness on.
The room does not close, and you get to choose where it shows
This is the one nobody mentions until it happens. You work around the room, and you come back to where you began, and the last seam does not match the first.
The walls do not divide into a whole number of paper widths, so the leftover is whatever it is. That is not a mistake and no amount of care avoids it. What you can do is decide the direction of travel so the final seam lands above a door, behind where a wardrobe goes, or in the corner nobody stands in.
Paste the wall or paste the paper, and it is not your choice
Non-woven papers are hung by pasting the wall. Faster, cleaner, and there is no soaking time to get wrong.
Traditional papers are pasted, folded paste-to-paste, and left to soak for a stated time so the backing does its expanding before it goes on the wall. Cut that short and it expands up there instead, which is what produces bubbles and seams that open a day later.
The paper tells you which it is. It is not a technique preference and doing the wrong one for the material is behind most bad wallpapering.
Butt the seams. Never stretch.
Edge to edge, no gap and no overlap. An overlap leaves a ridge that catches the light down the whole wall and cannot be fixed afterwards.
And if a seam has opened slightly, do not pull the next length across to close it. Wet paper stretches willingly and then contracts as it dries, so a joint you tightened opens wider than the one you were fixing, and since you will have done it at every seam the whole room gaps at once. It is the same failure as stretching turf, for the same reason.
Corners get cut, not wrapped
Measure from your last length to the corner, add about ten to fifteen millimetres, and cut the length to that width. Hang it so the small excess turns onto the next wall. Then hang the offcut on the new wall to a fresh plumb line, lapping slightly over the turned edge.
Carrying a full width round a corner that is not straight creases it, and worse, it carries the corner's error onto the next wall as your starting edge. Cutting at every internal corner is what stops errors accumulating.
The pattern repeat is what you are actually buying
Before any of this, the quantity. A repeat means each length has to start at the same point in the pattern, so lengths get cut longer than the wall and the offcuts are waste you cannot avoid. An offset match costs more again.
The wallpaper calculator handles that properly, including why you do not deduct doors and windows the way you would for paint, and why the spare roll has to be bought with the others rather than later. For the walls that are getting painted instead, the painting guide covers the order and the wet edge, and the paint calculator the quantity.
Common questions
Where should I start hanging wallpaper?
From a plumb line you have drawn yourself, near the most visible wall, and never from a corner. No corner in a house is truly vertical, so a first length hung against one starts out leaning and every length after it inherits the lean. By the time you have gone round a room the pattern is visibly running uphill. Draw a vertical line with a spirit level or a plumb bob, hang the first length to that, and let the corners be wrong where it does not show.
Why does the last piece never match?
Because the room does not divide into a whole number of paper widths, and it never will. You work around the walls and arrive back at the start, and the final seam meets the first one at whatever the leftover happens to be. That is geometry rather than a mistake, and no amount of care prevents it. The only real decision is where the mismatch lands, so plan the direction of travel to finish somewhere nobody looks: above a door, behind a wardrobe, in the darkest corner.
Should I paste the wall or paste the paper?
Whatever the paper says, and the difference is the material rather than a preference. Non-woven papers are designed to be hung by pasting the wall, which is faster, cleaner and means no soaking time. Traditional papers need pasting and then folding and leaving to soak for a stated time, so the backing expands before it goes up rather than afterwards. Get the soaking time wrong on a traditional paper and it expands on the wall, which is what produces bubbles and open seams.
Do I overlap the seams or butt them?
Butt them, edge to edge with no gap and no overlap. An overlap leaves a visible ridge that catches the light down the whole height of the wall and it is impossible to hide afterwards. What you must not do is stretch a length to close a gap: wet paper stretches easily and then contracts as it dries, so a seam you pulled together opens wider than the one you were trying to fix, and because you will have done it everywhere the whole room gaps at once.
How do I paper around an internal corner?
Never carry a full width round it. Measure to the corner, add about 10 to 15mm, cut the length down to that width, and hang it so the small excess turns onto the next wall. Then hang the offcut on the new wall against a fresh plumb line, overlapping that turned edge slightly. It sounds fussy and it is the only way that works, because a full width wrapped round a corner that is not straight will crease, and the next wall will start out of plumb.
How do I get bubbles out?
Work them out while the paste is wet, from the centre of the length outwards to the edges, with a smoothing brush or a plastic smoother rather than your hands. If a bubble is still there once it has dried, it is usually air that was trapped rather than paste that failed, and a small slit with a sharp blade plus a little paste worked underneath will flatten it. Bubbles that appear across a whole length hours later are almost always a soaking-time problem instead.