Dip a small artist's paintbrush in the wallpaper seam adhesive, which is available at hardware stores. Apply the paste to the back of the loose paper. Try not to fold the paper back as you apply the paste.
Smooth the paper in place and use a wallpaper seam roller to firmly press the seam down. A seam roller is a small wide wheel with a handle, it looks like a tiny, short paint roller. Roll the wheel along the seam, pressing firmly. Start rolling above the glued area and continue rolling several inches below the glued area.
Overlap the printed side of any torn paper over the side with the white edge. If there is a tear in the edge of the paper, glue the side of the tear with a white edge down first. Place the side that has color all the way to the edge over it. This will keep the tear from showing.