Here's a little project that is a couple of years old now. I always wanted a unit of sniper scouts to run with my Dark Templar army, but there were only two metal models available. Having painted up a version of each, I wanted a couple more that looked different and preferably kneeling down.
The first conversion was quite straightforward, sculpting a hood on the current kneeling model – easy enough. The second conversion was my first serious go with greenstuff. I hacked up one of the standing models, leaving just his torso and his boots. I pinned them onto a base and proceeded to fill in the gaps with greenstuff, using the current kneeling model as a reference guide. Once the legs had cured I moved onto the cloak, again using the kneeling original as a guide.
The one thing I did learn from this was that you have to let greenstuff cure slightly before attempting to sculpt it at all, then go back every half/hour and refine and smooth, refine and smooth – right up to the point where the thing is solid.
All in all I was quite pleased with the way these turned out. Trouble was, the following month GW released plastic sniper scouts – ouch!