I've used green stuff , pro-create and liquid greenstuff. It is incredibly easy: You get you casting putty and place it in to some boiling water for a few minutes. Please be careful when removing it from the water as its very hot! I use tweezers to pull the stuff out onto a flat surface (I usually put it on my cutting board but I'm lazy).
Anyhow, after that place the item into the caster putty and leave for 10-15 minuates to dry/cool down.
The casting puty can be bought of ebay very cheaply; once you see the results and can work it out yourself you be sooooo happy.
When it is cooled you apply water to the cast so the greenstuff doesnt stick. Then push the greenstuff in to the cast quite hard to make sure you dont get air bubbles.
Here is a few photos of my tools I used. There's also a a cup of hot water too, but forgot to put that in there.
Once the greenstuff had set I popped out the cast. After cutting the access off and smoothing things down you end up with the fist on the left.
Out of all three the products the green stuff worked the best but the pro-create was good but the detail wasnt as good. The liquid greenstuff is a total fail and just didnt work.
I'll be putting these casts, cut out, onto my Contemptor Emmure later on. Thanks for reading.