Create a simple and impressive giant cobweb out of white cord -- and don't forget to dangle a plastic spider nearby for the full effect.
Source: Martha Stewart
Thick white cord
Large screw eyes
Small screw eyes or removable adhesive hooks
Make vertical spokes: Insert a large screw eye at desired starting point. Mark 4 to 6 endpoints opposite screw eye and insert small screw eyes or apply removable adhesive hooks at endpoints. Measure distance from starting point to each endpoint and cut lengths of thick white cord slightly longer than each measurement. Tie each length of cord first around large screw eye and then at endpoint.
Make horizontal lines: Cut 4 to 6 more lengths of cord, long enough to span web horizontally at several points. Starting with the shortest piece, tie cord to vertical spokes near top of web, knotting at each spoke. Trim excess. Repeat with remaining lengths of cord, increasing distance between pieces as you move from top to bottom.