To make these, lay a trash bag out flat. Trim off the side and bottom seams so you’re left with two big rectangles of plastic. Cut each piece into an approximate square (eyeballing it is fine.) Each square will make one spider web.
Fold one of the squares in half diagonally to form a triangle. Fold that triangle in half two more times to narrow it. This is slippery stuff, so use a few pieces of tape to fasten the layers together where the loose folded edges come together. It will help hold the shape together when you’re cutting. (If any of the tape turns out to be on the web itself after you’ve cut it out, remove the pieces before unfolding or the web won’t be able to open up.)
Use a black Sharpie to sketch some guides on the plastic: draw a “stem” from the folded point out to the end of your triangle. Then sketch some scallops that come off the stem, like in the diagram below. With a scissors, cut out the negative shapes and unfold your web. Tape several webs together and add extra strips of plastic to connect them if you like.