I've just had my first go with Oyumaru. It really is as simple as all the videos say.
1. Get some Oyumaru:
2. Put it in hot water for a few minutes and it turns malleable. Press it over the object you want to copy (in this case, a 1/72 German Cavalry figure from LW):
3. Let it cool and harden, or run it under cold water for 30 seconds to harden. Remove the object. Mold is made:
4. Push Green Stuff into the mold. When the Green Stuff has hardened, remove it from the mold:
I know I used too much Green Stuff, but it'll trim off. Just look at how much detail it captured!
Very pleased with this.