Things like spider webs are surprisingly easy to make.
For this window decoration, I used:
1 1/2 packets of white Sugru (a certain kid decided to bite some of my Sugru packets, and the tiny holes cured half of what was in them)
less than 1 packet of black Sugru, preferably uncured (but I used cured stuff... see above)
scissors if your black Sugru is already cured (in case you have naughty kids at home too)
Step 1: Prepare the Sugru
Open the white Sugru packets (it's easiest to use scissors) and knead the Sugru with wet fingertips. Divide the white Sugru in half. Set one half aside and divide the other half into several smaller sections.
Don't open the black Sugru until the white part of the web is complete.