Remove all of the items from the room. This includes furniture, pictures and any items in closets. This is to prevent having to enter the room after it is closed.
Close all of the vents and, if possible, shut off power to the room.
Use caulk or sealant to fill cracks and holes around the windows and throughout the room. Follow the instructions for the specific solution that you are using.
Put plastic sheets up on the windows, using staples. Make sure you staple around the entire window; do not worry about how ugly it looks.
Put up shades and dark curtains and close them completely so that light is unable to enter the room.
Put plastic sheets up around the doorway, using staples. Remember to do this standing on the outside of the room. Make sure the door can close. Staple around the entire doorway. Secure the plastic sheet to the floor with tape or staples. Close the door.