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I've done some scratchbuilding myself, and am nearly done scratchbuilding a pair of ogryn.
I shied away from scratchbuilding proper for quite some time before I finally gave it a whirl. I had seen a lot of botched scratchbuilds, and I didn't want to make something I wouldn't be proud to have people see.
When I finally tried it, I was honestly kind of surprised with how easy it was. It was easy, however, because it really just pulled in all of the different techniques I'd learned doing a whole bunch of conversion work. I already knew how to do everything (except the face and hands), and it was simply a matter of doing everything at once.
So, the first thing to consider is what skills you already have with
GS. These will seriously determine what you do for your first scratchbuild. For example, I already had some experience doing commissar greatcoats. As such, I had the option of my first scratchbuild being a guy wearing a robe.
Secondly, I've found that you get a lot of mileage out of having a direct, physical source of inspiration. Making my first face went a lot easier because I had a little
GW cadian head that I could look at. Obviously, I didn't make an exact copy, but it was very helpful. Likewise for things like folds in clothing, etc. Needless to say, what kinds of bitz you have available will determine what you can mimic.
So let us know what experience you've already had with
GS, and what you have lying around, and we can help you a little better.