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I need advice on how to use Green Stuff. Do I just mix the colours together and mold it? How long does it take to cure? Any other stuff I should know about it?

Mix the two colours together in an equal amount untl it is all one colour. When working it you need to have wet hands, just have a pot of water to dip your fingers in (some people swear by saliva as a lubricant but I prefer water).

Most of all have fun with it

What he said, it takes 2-6 hours to dry but 24 to be hardened.

Once hardened it has a texture a bit like very hard rubber.

There's a lot of stuff to know about greenstuff. I suggest googling because there's a lot of tutorials out there already and then come back and ask more specific questions.

Some useful links from my 'favorites':

http://www.dakkadakka.com/dakkaforum/posts/list/242387.page

http://www.bananaking.net/cento/workshop/conv_green.htm

http://fromthewarp.blogspot.co.uk/2012/06/how-to-sculpt-greenstuff-cloaks-and.html

http://stores.ebay.co.uk/Green-Stuff-World-Online?_trksid=p2047675.l2563
[Note:not my ebay store... but some good stuff and prices]

The manufacturers recommend 1.5 parts yellow to 1 part blue (3:2 ratio) for general work. See here. Also see the additional links there to basic user and sculpting guides, should be useful to you. The extra yellow makes the mix stickier, less stiff, and longer-working. It reduces the springiness of GS, which can make it a little more difficult to make and keep fine, sharp edges.

The reaction between the two parts is affected by temperature. If you've got warm weather or live in a warm part of the world, or just have the central heating turned up, it'll harden a bit faster. Slower if it's cold. You can use this to your advantage: warm the piece up to make it cure faster (not too hot or it'll bubble and sag), or store freshly-mixed green stuff in the freezer to keep it workable for a couple of days.

Lubrication is useful, although I'd say it might be more useful on tools than on fingers. If you twirl the two colours between your fingers rather than squeezing, it'll prevent scraps sticking to your fingers. When it's partially mixed and more green than yellow & blue, it'll be safer to fold and squeeze.
There are several choices of lubrication. Oily types like vaseline (petroleum jelly), baby oil, or even T-zone grease from your forehead and nose. Plenty of people swear by them, although they often need to be washed off before adding another layer. Water is the other choice. I prefer that myself, but you need to be careful not to use too much, which'll flood round the putty and prevent it sticking to whatever you want it to stick to; or too little, which'll let it stick to your tools. I like to control the amount by slightly wetting a piece of sponge (like blister sponge), keeping it in a small, shallow dish, and dabbing the sculpting tool on that.

Tools... there are some choices there too. I prefer a metal tool, like a dental wax carver. Something with a rounded end and blunt edges to act like a miniature 'fingertip', pushing and smoothing, and making blunter marks; and a relatively sharp 'blade' end for making finer marks, cutting, and also a little spreading and smoothing.
Games Workshop tools are a bit like that, but are all blunt edges. You need something better. You can get cheap sets of wax carvers from ebay, Army Painter etc., but IMO the edges can still be fairly thick or roughly machined on these. You can get higher-quality wax carvers intended for dental work, through retail, and the good thing is these aren't too expensive either. I've been able to get a few off ebay UK for about £2.50-£4 each. You'll likely only need one for 90-95% of sculpting, if it covers the bases.
Certain types have their own names that you can search for. Among others, try looking up:

Zahle carver
Lecron
Hylin carver
Vehe
Beale carver

See how most of the variations have both the 'blade' and 'fingertip'.

You can ignore wax carvers, if you want. Tom Meier, a veteran miniatures sculptor (the veteran miniatures sculptor), uses a blunted hobby knife; a curved, filed-down sewing needle; and a small, homemade wire loop tool. Some sculptors use wooden tools, shaped from toothpicks, bamboo items like skewers and chopsticks, and boxwood tools for larger sculptures. Ball-ended tools for paper embossing can be useful for some jobs.
Clay shapers and colour shapers are fairly recent but good additions to the toolbox. They're like paintbrushes with a silicone rubber tip rather than bristles. They come in different sizes, shapes, and degrees of softness. Personally, I think that even the firmest type - the black clay shapers - are still a bit too soft to push hardening green stuff around, and too blunt to mark sharp details in it. They are very useful for putting the final smooth and polish on green stuff, though, which can be a problem, especially for beginners. There are a couple of small types that are double-ended with a wire loop tool, which makes them more versatile, but IMO they still need a bit of blade backup. YMMV.

Mixed green stuff is a little like chewed gum or sticky tack. When you're putting it on your mini, or WIP sculpt, it helps to rough up or mark grooves into the surface you're sticking it on. Give it something a little extra to cling to. Aside from that, try to push and shape the putty with a lot of small movements, rather than forcing it with long pushes, which have a greater chance of sticking the putty to the tool and pulling it away from the mini. Make and smooth the big, general shapes while the putty is fairly soft, then shape and mark the finer details when the putty starts to firm up. Firmer putty = less chance of throwing the details out of whack from little trembles or twitches. Play around with it, make basic shapes and textures, see all the little things that it can or can't do for you, and just how hard or gentle a touch you need. See the specific board here, the child boards and sticky topics.

Thank you guys. I managed to sculpt a few test purity seals and a hood which turned out well. I attempted to sculpt a waist cape but had to scrap it. The advice helps.