Preparation, preparation, preparation! It's easiest to work in an empty room, so if you can, move all furniture, furnishings and objects out of the room.
Cover existing flooring or carpets with dust sheets to protect them. Strip off all old paper and ensure walls are smooth and dry. If you've used a steam-stripper you may need to leave your walls for a few hours to thoroughly dry out. Fill in any cracks and holes and sand the filler when dry to leave a smooth, bump-free surface. Newly plastered walls will need to be 'primed' or 'sized' to ensure the new wallpaper will bond properly to the walls. The best way to do this is to brush watered-down wallpaper paste all over the walls to be papered. Leave to dry thoroughly. If there's any painting to be done, such as skirting boards, dado rails and door frames, now's the time to do it before you start wall-papering.
Preparation, preparation, preparation! For a top quality finish, it's recommended to line your walls first before applying your wallpaper. A heavy grade lining paper (1000gsm) can help to cover any imperfections in your walls and give a smoother final finish. Lining paper should be hung horizontally around the room so that vertical seams don't show through on the wallpaper. Don't overlap the paper but leave a small gap of no more than 2mm between joints when hanging lining paper horizontally. Leave to dry thoroughly before hanging wallpaper on top of lining paper. Any initial air bubbles in the lining paper should dry out and disappear. If any persist, make a very small cut in the lining paper and brush in some wallpaper paste and smooth back down. Allow to dry.
Measure twice, cut once is the old adage! Don't assume all your walls are exactly the same height either, older houses in particular are liable to have 'settled' over the years. Measure regularly as you go. Allow 50mm top and bottom (100mm in total) to each piece for trimming at ceiling and skirting board. Measure your first length of wallpaper and mark it on the back with a pencil and straight edge. Cut with wallpaper scissors or a sharp craft knife and metal ruler for a perfect straight line. You can now use this piece of paper to measure against the remaining roll for your next piece of wallpaper (don't forget any differences in your wall heights). Also, if you have a pattern repeat, you'll need to check whether it's a straight match or if you need to allow enough extra length to match the pattern. Once you've cut your paper, number each piece and mark the top and bottom to avoid hanging any pieces upside down!
Depending on the paper you've bought, you'll be either pasting the back of the wallpaper (traditional method), soaking the wallpaper in water (ready-pasted), or pasting directly onto the wall (new paste-the-wall papers). Assuming you're pasting the paper, work down the length of the paper and from the centre to the edge. Don't paste from the edge to the centre as this can get paste onto the front side of the wallpaper. Once you've thoroughly pasted the paper, gently fold the pasted edges together (don't crease it!) and leave it for the recommended time as stated on the label. Some wallpapers need to 'soak' for a short period, this is to allow them to stretch before applying to the wall.
If your wallpaper has a bold pattern, you should see if there is a central focal point in your room such as a chimney breast. If so, hang your first piece exactly in the centre of this feature and then work out from either side. If you have a plain patterned wallpaper or no specific focal point, it's best to start from one side of the window (usually the right hand side) and then work around the room into the furthest corner. Then work from the other side of the window into this corner. Before hanging your first piece, you must draw a vertical line on the wall using a plumb line. Hang your first piece against this line and you'll have a nice straight edge to butt your next piece of paper against. Trim the top and bottom with wallpaper scissors and smooth down with a paper-hanging brush making sure there are no bubbles. Hang your next piece butting up to the edge of the first, don't overlap or leave any gaps, run a seam roller over the join.
Don't try and hang a full width piece into a corner, it's better to hang it in two pieces. From the last piece before the internal corner, measure from the paper into the corner and add approximately 25mm to this width. Cut the paper down the length to this width and paste into the corner. You should now have a 25mm overlap on the next wall to be papered. Mark a new plumb line on this wall and use the remainder of the last piece used to paper the new wall. Carry on with a new full width piece butting up against the join as before.
Similar to internal corners, but make the overlap onto the next wall a bit wider, 50mm should be sufficient. Match the pattern as best you can and then, to ensure a butt joint, slice through both pieces of paper (with a sharp knife), peel back the top paper and remove the piece from underneath. Smooth back the top paper into position and you now have a butt joint.
Better to be safe than sorry, so switch off the electricity at the fuse box first. Smooth down the wallpaper over the switch or socket, find the centre and make a series of diagonal cuts towards the outer edges of the plate - don't cut further than the edge of the switch plate. Fold back the triangles of wallpaper you've just cut and trim off most of the wallpaper. Loosen the screws in the switch plate so you can pull the plate away from the wall and tuck in the edges of the trimmed wallpaper. Tighten up the screws for a nice neat job.
If wallpapering the ceiling and the walls in a room, it's advisable to do the ceiling first (so you don't get any mess on your newly papered walls!). Work across the room, parallel to the window wall and paper from the window into the room. You'll need to mark a guideline across the ceiling that's parallel to the wall to give you an 'edge' to work to for the first piece. Work with the 'folded' pasted wallpaper across one arm and gradually 'unwrap' the folds as you smooth the paper onto the ceiling. Butt join your next piece and work across the ceiling. Light fittings can be treated in the same way as sockets and switches. If you prefer to dismantle the light fitting, ask a qualified electrician if you are unsure how to do so and ensure you switch off the electricity at the mains first!