Apply black vinyl decals to the walls. Flatten out any air bubbles and tape the top of the decal to the wall with masking tape. Lift up the decal and peel away the paper backing. Press the decal against the wall to make it stick. To remove the decal, hold one corner and peel it off the wall.
Drape black curtains or sheets across the walls. Attach the fabric to the walls using tacks, nails or a staple gun.
Hang large squares of black poster board on the walls. Other types of paper also work, but poster board lets you cover larger areas quickly.
Apply black wallpaper to the walls. If you rent your home, ask your landlord for permission before wallpapering.