Hanging border with your wallpaper installation
If you want to hang the border with your wallpaper installation you will need to cut the border in. This method looks the best because your border is not on top of the wallpaper but spliced in at the top of the wallpaper. Hanging the border this way you will install your wallcovering to one wall of a average size room, and as soon as you are done with the wall, you will splice your border in before the wallpaper has dried.
Splicing in the border
Measure the width of your border. Then measure down from the ceiling (on a border hung at the top of the wall) a little bit less that the width of the border and mark it with a pencil. If you border is 6" you will measure down 4". Lightly draw a line the at the 4" mark all the way down the wall or if you have a laser level, simply level it and put it on your mark and cut off the top 4" of wallpaper using your level as your guide.
Since you are not going to hang the whole spool of border at once, measure out the amount needed to go the length of the wall plus a foot over. Cut, wet , and book this strip. After the proper booking time, install your border using the same procedure as mentioned in parts 1 and 2. Cut your corner in the proper manner at the end of the wall - cutting onto the next wall 1/8th inch.
Once the border is installed, you will go back and splice it in and remove the 2" overlap of wallpaper from underneath. This will give you a perfect and tight seam joint where wallpaper meets border. Using your 6" spackle blade, put it on the border and line up the bottom of the spackle knife with the bottom of the border slightly ( 1/64th of an inch) on the border which will give you a good double cut, tight seam, instead of slightly below the border. If you cut below the border, you will have a gap, and you will need to pull the border down to the wallpaper - this won't be as good a seam as a double cut seam.
Proceed on to the next wall and hang the whole wall and again immediately follow up with the border in the same manner. When you hang your border this time you will need to match up your pattern back into the corner that you finished on the first wall. Take your dry spool of border to the corner you will be starting at, now slide it over onto the wall you've just hung the border on and match up the pattern. Once the pattern is matched follow it to the corner and cut it straight down at the corner. Once the excess border is off , you should be pattern matched at your starting point in the corner. Now you can measure your border (the length of the new wall plus one foot excess) and cut, wet, book, and hang it.
Continue this procedure around the rest of the room and you will have a nice spliced in border that will look great.
Hanging border directly over wallpaper
This is the most common method of hanging border in conjunction with wallpaper. You should not do this over wet wallcovering. Let the wallpaper dry good and dry all the way through. This may take a week or two depending on the type of wallpaper that was installed.
Hang your border exactly as you would on a painted room, except you won't wet the border. In this situation you will be applying what is called vinyl on vinyl wallpaper glue directly to the back of the border. Allow it to book and then apply it directly over the wallpaper at whatever location you desire.
This method does not look quite as nice as splicing the border in, but it is admittedly simpler.
Mitering border joints
One other item should be addressed here and that is how to miter border joints when you frame a door way or window etc... with border.
To frame an object with border you would simply frame the object with your border, overlapping and over extending at the ends. Then the only real rule with the miter cut is to cut from the inside corner where the two papers meet to the outside corner again where they meet. See figure below and the 45 degree cut. As a rule you will cut straight through. You may get a noticeable mismatched somewhere along the 45 degree cut, if it looks too objectionable, you can cut out a piece of the missing pattern off of a scrap piece and glue (vinyl on vinyl) it back on the mitered cut. If you trace the pattern nicely, you won't know it has been placed back on top of the border.