More or less the same way it got onto it! Try an damp cloth and a hot iron over it and it should loosen off so you can pick it off - might take a while but it ought to work.
I don't know about using a damp cloth, but you could try using a (dry) kitchen paper towel, or brown paper, rougher side down, and applying a hot iron. The Wonderweb should stick to the paper rather than the fabric.