If your home contains windows that are not trimmed with moulding, then wallpapering requires tucking the wallpaper inside the window frame in order to maintain continuity of the papered wall. While papering inside a window frame requires a little more effort than papering around a window with moulding, hiring a professional is not necessary. If you fail to properly prepare and place the wallpaper inside the window frame, however, the wallpaper may peel or lift.
Measure the room for wallpaper and place the first sheet of wallpaper in a position such that when you arrive at the window you will have enough width on the sheet to fit around the wall and into the window opening.
Place the sheet of wallpaper that will encompass the window flat on the wall and smooth it in place as you did previous sheets.
Use a utility knife with a new blade to cut a slit in the wallpaper from the top corner of the window frame along the top of the window frame. Repeat the process for the bottom of the window.
Tuck the wallpaper around the edge of the window and into the window frame. Make sure to press the wallpaper flat into the frame, especially at the top and bottom edges.
Trim the excess paper on the inside of the frame with a utility knife.
Place the next sheet on the wall and repeat the process on the opposite side of the window.
If the wallpaper inside the window frame lifts a bit after installation, use wallpaper glue to fix it back in place. Cutting narrow strips for the inside of the window frame will likely result in lifting and peeling.