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.
Step 2: Make the Spider Web Spokes
Repeat this with another tiny bit of white Sugru. This time, place the line on the window so the midpoint is crossing the midpoint of the first line.
Keep going until you've made as many spokes in the spider web as you'd like.
Make sure to dip your fingertip in the water before pressing the Sugru rope against the glass, or you'll be swearing a lot as your delicate spider web sticks to your fingers and pulls away from the window. Make sure you don't use so much water on your fingers that it drips around the edges of the Sugru ropes and coats the window; that would keep it from sticking to the window.
Step 3: Add the Spiral
Start in the center of the web near where all the spokes intersect. Gently press the end of the long rope of white Sugru into one of the spokes to hold it in place. Carefully spiral it around and around, working outward. Press the Sugru gently against the glass and the previously laid ropes of Sugru.
When you've used all the white Sugru and your web looks decent, take a tool (I used the blunt end of an xacto knife), dip it in water, and GENTLY push the sections of the spiral line inward toward the center where they aren't attached to the spokes. You'll want them to curve inward just slightly.
Step 4: Add the Spider
If your Sugru is cured already, slice 8 legs with a scissors, then cut a body shape with the head attached. Stick these to the spider web before the white Sugru cures.
You should be done now. Thanks for reading!