When installing wallpaper behind a fireplace, it is important to choose a heat-resistant wallpaper or non-combustible material to prevent the risk of fire. It is recommended to leave a gap of at least 1 inch between the wallpaper and the fireplace or any heat source. You may also want to consider using a wallpaper liner or backing to help protect the wallpaper from the heat. When installing the wallpaper, use a sharp utility knife or scissors to carefully cut around the edges of the fireplace and take extra care to avoid any wrinkles or creases.