Once your bay openings are finished inside and out, you can install the bay walls.
Finishing them is tricky. They do not have much accessibility so it might be easier to do it in the following way.
I first measured out the wallpapers height and width by wrapping it around the exterior of the bay instead of trying to grapple with it through the interior.
Once you have the piece needed, cut to the right size, cut it again in half. This will make installation much easier.
Apply the bottom half first. Make sure you tweak the fit before you glue it on since measuring from the exterior will not give you a perfect size.
If you accidentally cut too much, don't worry, just patch it. Most of the bay walls will be covered with window trim so don't panic.
Once the bottom is up, apply the top. Tweak the fit in the same way as you did the bottom portion. Butt both halves flush together. They should fit like a puzzle. There is no need to overlap them, just make sure they are flush.
Making the cut in the middle is the best way to join the two halves seamlessly since most of this area is covered by window trim.
Once the wallpaper paste is semi-dry, you can cut out your window openings. Don't wait for the paste to be completely dry, as this will make cutting out window openings harder but you don't want it to be completely wet either, since it can tear your wallpaper.
Keep In Mind
Push your wallpaper tightly into all corners and the joints between the bay walls. Do not leave any area pocket in these areas since they will make installing the window trims very difficult.
Use tacky glue to install this wallpaper instead of wallpaper paste. Tacky glue dries very thin and flat. You will need your wallpaper to be as flat as you can get it since the window sills of these bay windows are installed in an interlocking fashion that will be extremely difficult to if there is any thickness between the wallpaper and the wall. Wallpaper paste tends to add a little bit of thickness to the wall it's applied to. It also globs a little along joints and you do not want that to happen with these bays.
Because the wallpaper is applied in sections, you do not need to worry about the tacky glue making its application more difficult. Once you have each half fitting perfectly, you will not need to shift your wallpaper around for a better fit.
Use a heavily patterned wallpaper for this room. You will need for the patterns to hide any patches or inconsistencies.
