Sounds like you're reffering to 1/48 or 1/72 figures. They are harder to sculpt than 1/35 (about 5cm tall) so if you do want to try your own sculpting, it would be a good idea to start with larger figures.
To begin with, sculpy is not the thing to use - it's basically glorified play-doh. What you want is a two-part epoxy putty like Milliput or Apoxy Sculpt - epoxy is smoother, easier to work, and cures much stronger than any play-with clay will. Best of all, epoxy putty is water-soluble before it cures, so you can wet your tools to make your job easier.
You'll need special tools to sculpt fine detail - I use a dental pick with one flat end (similar in shape to a flat toothpick) and one rounded, pointed end. A cosmetic orange stick sanded to shape also works. The good old #11 blade isn't that useful for sculpting, so if you were trying to use one of those, it's no wonder it isn't working.
Faces are about the hardest thing there is to scuplt, I'd recommend using pre-made heads until you get very good at sculpting - even I can't do very good heads yet. It is all done by hand in most cases, so there's really no quick and easy altenative.
Plastic figures are made by injecting a metal mold with liquid plastic, than adding a hardener to make it cure. Resin figures are made by pouring a two-part epoxy resin into a rubber mold. Since the rubber is more flexible than steel, figures cast in rubber molds are usually better detailed and in fewer pieces because the rubber mold can have undercuts. You can buy casting resin and RTV (room temperature vulcanizing) rubber online or at good hobby stores, however a good starter kit will cost around $50 and only contain enough rubber to make molds for about 7 figures. The molds must be designed very well, too, or they won't work.
I think your best bet right now would be to buy some plastic figures and practice modifying them using epoxy putty. You should learn how to sculpt well before you attempt an entire figure from scratch, because then you not only have to worry about the details, but all the proportions must be exact as well.