Adding a border to your freshly decorated room can be done quite easily, a lot of wallpaper manufacturers also produce borders to compliment their range of wallpapers and a border can enhance even a very plain decorated wall.
First decide on a suitable height for the border and mark this measurement around the room using either a tape measure and a long spirit level, or if you have one a laser level, any height levels should be marked in pencil so that they can be easily removed later if necessary.
The strip of border on each wall should if possible be in one complete length to save you trying to match up the ends. Use a good quality border adhesive to make the task easier, lay the border face down on a wallpaper pasting table and apply the adhesive to the back of the border with a 2″ (50mm) paint brush and fold it back on itself in a concertina fashion (see post on pasting and hanging wallpaper), then following your level marks on the wall using a clean sponge or wallpaper brush, stick it in place. Trim any ends of the border where it meets up to woodwork and fireplaces etc, as you would with a strip of wallpaper.
Remember to wipe away any surplus adhesive before it drys.