Well, I'm far from an expert, but I've been dabbling for a few years, so I'll try to get into a little bit of detail on how I do it (probably the real sculptors will laugh at that). The most important part for me is work in layers and don't try to do too much at once as you'll only end up damaging one bit while working on another. Instead, let every part you did cure before moving on to another one. you can work on a few figures like that every evening or whenever you have the time.
The tools I use are:
- toothpicks (for some detail and for smoothing out the putty by gently rolling it over)
- needle pin for finer detail
- scalpel tool for cutting (like the sleeves)
- clay shaper (with a soft rounded tip, also for smoothing)
Keep a cup of water handy to wet your tools or the putty will stick to them! Pro's use thinks like vaseline or saliva for a smoother finish.
Mixing the putty: more yellow makes it more sticky and gives you a longer working time. more blue makes it less sticky, cures faster and holds detail a little better.
To give an example of sculpting, the puffy sleeves are achieved by first putting a blob of putty on the arm, then smoothing it out around it (using the clay shaper or the toothpick) until the arm is covered on all sides by a sleeve of putty. This is then cut vertically using the scalpel (don't draw it across the putty, but press it into it). If you want it to look like on the last figure I posted, repeat with horizontal cuts. I then use the tip of the toothpick to push in the sides of the different putty squares (the vertical ones) and give that extra touch if you can see what I mean in the picture. If you only made vertical slashes, you can do the same with the scalpel blade. Pants are similar (though only vertical slashes).
When you've done one sleeve: stop. let it cure and work on a different model. If you try to do too much at once, you'll end up putting a finger in your freshly sculpted detail.
When you've done the pants and they have cured, start with the jacket (or the shirt first if it's an open jacket). Don't try to do front and back at one, just one side will do.
Patience is key. And switch between tools to find out what works best for what purposes.