Preliminary tasks.
Read the border label for installation details such as: booking time, determine if your paper is pre-pasted or not, if not what type of paste is recommended? Note any other special instructions listed. Examine the border (each roll) to determine if it is defective. Are they all from the same run or are there color differences? Also be sure you have enough wallpaper to do the entire job. Determine your beginning and end point (which is the same spot usually - you will end where you begin unless you are doing less than all four walls). Your beginning / end point should be in the least conspicuous corner (or elsewhere if there is a better place to "hide" it) as you will have a mismatch at this point.
Priming.
I have a bias here. I almost always prime before hanging wallpaper on the wall or just border. I don't want to botch up somebody's house because I couldn't take 30 minutes to prime the wall. When it comes time to remove the paper and the wall gets torn up because it wasn't sealed well, believe me you will be getting a call from one upset homeowner if that happens. So don't take the chance. Prime the wall before hanging, it is a great rule to adhere to (no play on words intended here). Over a primed wall the wallpaper will install easier, potion easier and remove easier.
My favorite universal primer for wallcovering is Zinsser's "Shieldz". It is waterbased, dries quick and works in almost any situation. Shields comes in white, but can be tinted to approximate the background of your wallcovering to help hide seams on dark background wallcoverings.
To apply primer to a wall where you are only hanging border on, you will need to measure the width of the border and apply the primer from the ceiling line down a little less than that width to the wall. If your border is six inches wide, then you will apply the primer from the ceiling / wall inside corner down about 5 and 1/2 inches. Don't go down exactly the exact width of the border, if there are any wiggles and fluctuations in the installation (and there will be), your white primer will show below the border. You can brush the primer on or use a mini roller, whichever you prefer. Allow overnight dry if possible, but at least 2 hours before hanging your border over it.
Wetting (or Gluing) and Booking.
Set up your 5 gallon bucket and screen with clean tap water and place it by your work table.
Unroll 5' of your spool of border (with the glue side up) on your work table. Set a paper weight at the coiled end to hold it in place, and roll water (if pre-pasted) or glue if (un-pasted) onto the top side of the wallcovering.
After a 5' strip is wetted / glued, fold the section in half, glue side to glue side together, this is known as "booking" the wallpaper. Unroll another 5' section, set your paper weight down at the coiled end and repeat the procedure, booking glue side to glue side. Booking allows the wallcovering to "relax" and soften. During this period of time the wallcovering will swell or expand, we want this expansion to occur off the wall, so don't just wet / glue the border and slap it on the wall. Wait for the wallpaper to relax and expand while it is booked together - off the wall. Repeat this until the whole roll is done.
After the whole roll is wetted / glued, booked and the allotted book time is elapsed, you may begin to hang the border.
Hanging the Wallpaper Border.
Bring the border to the wall holding it in your left hand (if right handed), and apply the starting end to the starting point which you established earlier. Position it in place with your right hand as it un-books one fold at a time. Once a section of 5' or so is in place, smooth down the border and remove any air bubbles with your plastic smoother. Smooth in a up and down manner to avoid stretching the border which will occur if you smooth lengthwise with the border. Once it is smoothed down, rinse with clean water from your bucket the ceiling , wall and the border itself to remove any glue off them.
How I secure the spool so that I can move the ladder down the wall.
If you are working alone you will find that you can't hold the spool of border at the ceiling and come down the ladder at the same time. So what I do is I take a thumb tack and tack the coil of border to the wall to hold it in place so that I can come down the ladder and move on with the installation.