Ditch the water tray
Most DIY'ers activate the glue on their prepasted wallpaper by dipping a reverse rolled strip into a water tray. There is nothing more messy in by view than hanging prepasted paper with a water tray. You will end up dripping water all over the place.
The professional method of activating the glue on prepasted paper is to use a 5 gallon bucket with a roller grid and a roller. In much the same way as you apply glue to unpasted paper - you will apply water to prepasted paper - with a roller working on a paperhanging table.
Prepaste activator
All right lets go all the way. Water is thin and runny so you will notice that most professional paperhangers actually use prepaste activator for prepasted paper and not water. Prepaste activator is thick and clings to the paper better than plain water. Prepaste activator is very much like applying glue - no runny drippy mess.
How to activate your prepasted paper
Cut your strip and place it onto your table glue side up. Apply the activator with a roller to the top half of your strip and book it. Slide the paper over and apply the activator to the bottom half and book it.
Hanging
After allowing the prescribed booking time (see wallpaper label) , bring it over to the wall to hang it. Starting at the top let the wallpaper unfold and drop down, smooth the paper down as you work your way down the ladder.
Should I use a plastic smoother or a smoothing brush?
I confess I am not a true wallpaper "purest", I do use a plastic smoother more that I should admit to. The problem you can encounter with a plastic smoother is that they tend to stretch the wallpaper. If you should choose to use one, smooth the wallcovering "back" toward the already smoothed out area (and toward the center) to avoid stretching the paper - making it longer and/or wider.
The smoothing brush on the other hand is gentle and while it doesn't stretch the paper as the plastic smoother, you need to be careful to keep glue out of the bristles ( the plastic smoother will easily rinse and wipe clean if you should get glue on it). On delicate papers you must use a smoothing brush.
Trim the ceiling and base to size
Using your plastic smoother (or 6" spackle blade) as guide and a new single edged razor trim the ceiling overage and at the baseboard. Wipe off the glue from the ceiling and baseboard with a sponge and clean water. You have just finished hanging your first strip. Hang the second stripin the same manner ( I will discuss cutting, engineering, inside and outside corners, windows, doors, and many specific issues in separate articles).
Seams
Cut and activate your second strip, allow it to book, and then bring it to the wall, starting at the top of the wall line up the pattern and slowly unfold the paper working your way down the strip - smoothing it down as you go. Once the second strip is down and smoothed, trim the top and bottom in the same way as the first strip. Next you want to "fix" the seams by rolling with a seam roller , be sure the seams are not overlapped or gaped as you do your final roll. Wipe the seam with clean water and a sponge, and wipe the ceiling and baseboard also. Continue on down the wall in this same manner.