Draw three different-sized spider webs on the Mylar sheet with a pencil, using the ruler. Draw a spider web by drawing a cross shape and drawing diagonal lines to create an "X" shape over the cross. Draw curved lines between the cross and "X" lines.
Place the Mylar sheets on the cutting board and cut out the spider web designs with the craft knife, leaving small pieces of Mylar in place along the design to hold the stencil together.
Paint the walls with the gray paint, using the paintbrush and paint roller. Let the walls dry for three hours.
Tape one Mylar stencil to the wall with the painter’s tape. Apply white paint to the spider web stencil with the pouncing paintbrush. Remove the stencil. Repeat, stenciling the different-sized spider webs over the walls in a random pattern. Let the white paint dry for three hours.