I made them with instant mold, a silicone based mold that is soft under boiling water and gets hard as it cools off. It can be bought on ebay for very low price.

I used my NECA Alien figure for the nice details. The side of his head, his chest, base of the tail and his leg were press molded and then pressed onto a pile of sculpey (green stuff to be exact) to make a positive impression. It's a quick and dirty job and the effect depends on the stuff you used to make the negatives of.
The process:
1) Boil water in a kettle
2) Pour the boiling water into a bowl and throw in the instant mold (watch out! Hot!)
3) Wait 2-3 minutes till it's soft and guey
4) Make a 1cm flat pancake and press it onto the desired thing you want to copy (just try to copy things that are not too 3D, or you won't get a nice negative)
5) Wait for it to cool off, sip some tea (10-20minutes, depends on the amount of mold used)
6) Carefully take off the mold
7) Prepare some Greenstuff
8) Put it on a surface you want to make the impression.
9) Use a bit of water on the mold for easier release (greenstuff tends to be sticky, although instant mold is near impossible to stick onto)
10) Press down the mold, press it moderately hard.
11) Wait 10-20 minutes and then carefully take off the mold
12) Wait 2-3h for the Greenstuff to harden
13) ...
14) Profit?!?
That's about it

. Steps are very easy and you can reuse the mold many times!
If you make 4-8 different, smaller surfaces you can mix match them to make a unique effect!
Here's the hero of the project

