Hi Librarian,
I agree with everything Mordheimer has said, plus...
If you can get it try procreate (grey stuff) - I've found it less sticky to use than green stuff and it was cheaper.
As well as water, WD40 (or similar) sprayed on to the green stuff makes it easier to work (but do this in an open area).
Also scibor has some superb tutorials on his website (the man is a sculpting genius).
Just try stuff out - its great fun to get creative - love the spawn by the way - it's a really dynamic pose you've created and the greenstuff looks pretty organic and the commissar is brilliantly fluffy Mordheimer!
Here are a few of my efforts...
Daemonhunter Grand Master (Cloak and resizing body)

Blood Bowl Thrower (complete sculpt from scratch - used a reaper miniature armature for this one)


My Vulkan H'estan (sold on ebay - because were moving soon - got to save some cash and make some room) - scaly cloak, lava and cloth strips

Just have fun with it!
Laney