Aye, what Dalin said, but I'm too lazy to click on videos.
Green stuff isn't as claylike as
DAS. It's got more of a stretchy, chewing gum/blu-
tac consistency. More on that later.
So how do you use green stuff - is it just shape it and leave it to dry
Pretty much. You can work with that process too - bung it on when it's freshly mixed and stickiest (scoring or roughing up the mini's surface helps too); shape the main masses when it's softest; and if needs be, refine details when it starts to cure and firm up, when you can be a bit less concerned about twitching in the wrong direction and messing things up.
how long does it take to dry etc?
Depends on how much of each part you mix and how warm your surroundings are. More blue for a faster cure, more yellow for slower. Warmer temperatures speed up the process, cooler temps slow it. It might stay workable for as little as 40 minutes, to two hours or more. Speed-cure by tilting an incandescent desklamp over it, or applying some other source of gentle heat. Keep it workable for a day or two by sticking it in the freezer.
What's it good for, what's it not so good for?
It's generally a good all-rounder. Being elastic means it can be worked into very fine details without tearing or crumbling like some more claylike media. If you've used
DAS I'm sure you know a bit about that!
It can be a disadvantage too, as green stuff is the sculpting putty with the most 'memory', causing it to 'spring back' to some degree, especially when it starts to harden. This means it can be easier to achieve sharp edges and e.g. mechanical details with other putties. (brown stuff, with virtually no memory, used to be the go-to putty for that kind of thing, until sculptors started trying others and the manufacturers pulled production.) However, that's with a basic 50:50 mix of yellow:blue. The manufacturers started to recommend a mix ratio of 1.5 parts yellow to 1 part blue - or 3:2 - which gives a much softer, more 'fluid' standard, and negates some of the memory problems.
After curing, it's fairly rubbery. You can cut and carve it, but it doesn't really take filing or sanding very well.
If I wanted to do something like my own Space Marine shoulder pads, is green stuff the right tool for the job, and can I make molds for it?
Shouldn't be too bad. You might want to really push it into some of those sharp corners of the mould, though. I don't know if it'd ever be just as crisp as more liquid casting media.
At £7 for 20g, is it good value/worth it?
Oh dear goodness, no. Like Dalin says, you can get it in 36" 100g strips for almost the same price. Also in 100g tubes, with individually wrapped bars of blue and yellow. The advantage with the latter is that the strip starts to cure in the middle, where the two parts touch, causing hard lumps in the mixed putty or other curing problems. Usually a good idea to cut out a few
mm around that meeting area and discard it, and that's a fair bit of putty in the bin, right off. The tube avoids that problem. Though the disadvantage is that the bars of the tube are equally sized, and the strip normally comes with more yellow for the recommended mix ratio. That isn't too much of a concern
IMO - sometimes you want more blue.
Dark Angels/Deathwing - just getting started!
Space Marines - 
Eldar - Biel Tan 2000pts
