First work out where you want the border to go, then with a spirit level to get it straight (the bubble is in between the 2 lines, draw a horizontal line around the room.
To continue the line across a door or window frame, get a wooden batten which spans the width and holding it level with the line on one side, mark the other side.
If the line is accurate it will meet up where you started.
Hey presto, all ready to go. If you are putting up the border onto vinyl wallpaper, use a 'border adhesive' as ordinary paste won't stick. If it's going onto paint, wallpaper paste will do, unless you're using self adhesive!!
Good luck