OK, there are much more professional sculptors on this forum, but I might still throw in my 2 pence for a starter.
Kneadite (or kneadatite), AKA Greenstuff is OK to start with. Other mediums work a bit differently, and have different qualities when drying/dry, but since you will be learning first, just pick any two-component medium you can find easily. Pick equal parts of both components, and knead them together so long that they form a completely unicolour mass. Be sure to pick out any hard bits (greenstuff sometimes has these where the two components touch each other in the box), and hairs etc.
You need two tools at the minimum: a flat/curved "knife/spoon" type of blade to finish off flat surfaces, and a sharp point (like toothpick) for the fine work. Metal tools are always better, although you could improvise with plastic, even some bits. Professional tools are available, and the one GW tool or ArmyPainter etc. are not bad. Dentist's tools might be the best, when you have enough skill & precision to benefit from the micrometric accuracy.
A photo, a drawing, or perhaps a 3d model are handy when searching for a form for the model.
The body is sculpted on a skeleton of wire (see below). Take time to bend the wire into a (dynamic?) position you are looking for. Think about what kind of angles and crevaces you are creating, how complicated it will be to get the scuplting done in places hard to access, etc. First use blue tack or childrens' hobby materials (in Finnish that's "molding wax") to make a rough design of the sculpt to see where you will be building up mass, and how the figure might find its position & balance. Be sure to wash the wire skeleton after these mediums, as it will get sticky and greasy. Overall, keep your tools clean throughout the process.
Greenstuff is molded with wet tools, so that the stuff doesn't stick to the tools. So you need a glass of water at the minimum. You can also use more professional mediums, I guess some sort of liquid silicate is sold for this purpose especially. There's also the yucky but fast way of wetting the tools in your mouth - saliva is quite good a medium for the purpose - but then you know you are actually eating bits of greenstuff. At your own risk.
Greenstuff stays very soft for 10-15 minutes. Use this time for sculpting the big forms. Then, as the stuff gradually hardens, it is time to add detail. Two things to remember: greenstuff "rounds up" a little bit while stiffening, so either work over the sharp & little detail you made while the stuff was softer, or leave some thickness so that you can file or sandpaper the sharp corners when the stuff has hardened. Secondly, greenstuff doesn't take a knife well, so don't plan to sculpt the stuff when it's hardened. Other sculpting mediums have a bit of the same - some are simply too hard to work with a knife after they've set (if you want precision), but at the same time these are even better for filing and sandpapering.
I would start humbly, which in my own case meant using GW bits which were pinned into the wire skeleton. I then filled in the body in between the head, arms & legs - first with blue tack for a prototype, and then in greenstuff. This allows you to study what greenstuff is, how it molds, how it dries and sets. You probably do 1-5 figures before you know the medium & your tools. So don't get upset or let down when the first test figures end up ugly. They always do.
The basic rule is to work in layers, to build up from the skeleton upwards towards the skin & clothing. (As you get comfortable with the stuff, you may try to "mold directly" the kind of surface you want, i.e. mold both skin, clothing, belts etc. from the same lump of greenstuff.) If the new layer of stuff doesn't seem to stick on the old one (that has dried for an hour or several hours), you can use sandpaper to increase the stickiness of the surface, or even use some glue, if you wan't to work fast. Greenstuff is plastic, so plastic glue works fine. Other mediums are clay-like, so require superglue.
Here's my first semi-successful sculpt:
This shows the original wire skeleton, some testing with blue tack, and the first layer of greenstuff that turns the skeleton into a more meaty body. Alas, I didn't take pictures while doing the surface & clothing, but here is the end result, painted:
The picture quality is not very good, but I hope you can see the basics. Actually, the dog is the fully scratch-sculpted piece, as the guy himself has GW head and arms.
There would be a lot to improve in this figure, especially smoothing the surfaces, adding folding, and making the legs a bit heavier. The painting could be much better, and it would have been easier if the figure had had smoother & better surfaces. All that said, I'm quite happy about having this figure that is usable in gaming and surely individual. A nice third try in sculpting that has its use.
When you have reached this level, the real work of refining you skills starts.
One way to study the greenstuff & get practice in fine work might be to continue with ready made stuff, and add details. You can find a lot of threads on this forum where people have made their own modification to figures. Ogres might be the most common topic, as people turn the present-day fat brutes into more sophisticated Empire-style Ogres. Essentially, all of that is sculpting on a semi-ready basic surface. Much of the refined sculpting will be exactly the same kind of job, even if you work over a sculpt made entirely by yourself.
Here, I've toyed with some Empire bodies and turned them into Greatswords:
You can see that not all stuff should be made out of greenstuff or other molding mediums. Where you want thin & smooth surfaces, metal sheets may work (here: pieces from a tin can were used for those vellum pages). Where you want straight poles, hafts, handles, weapons etc, use plastic rods and plasticard. With plastic(ard), it is often easier to work by carving and filing, from big chunks towards the goal - just like carving stone.
These are all the pictures I've taken from my sculpting attempts (that I wish to show here, haha). Maybe you could check Youtube for videos, as I know there are some. "greenstuff" and "sculpting" should bring you hits.
-Z
