I've put together a compilation of my process, sorry for the delay
1. original screenshot
2. first details (resizing the helmet from behind, drawing a completely new shoulderpad/neck guard, highlighting the lines that I want to show on the icon, giving him a bit of a facelift & a nosejob, etc) - I then resize it to 64x64 to see how the lines show and how everything works (this was one of my first icons so obviously I saw I had LOADS more to do)
Because I was at the time planning to use this for the crossbowman model, I decided I needed to change it up a bit so I used the crossbowman front helmet cover thing - and this is the part I'm worried about. It is two bits (circled on image 3) but I can try contacting the author of that model if that's necessary.
3. crossbowman screenshot
4. picture no.2 with the new helmet cover
5. final details before resizing
6. I then resized the icon and framed it to see how the composition of everything performs, and then I decided I didn't like the crossbowman cover - but I've done so much I couldn't just erase it, so I reduced it and changed the location of where light falls on the helmet. This is also the part where I do the most details to really make an icon - and not just a framed screenshot. The bottom two icons are the final icons; I was happier with the blue one so I uploaded that one as my icon of choice, and pinned up the red one because - why not?
Bwoop, took me a while but here I am! Thanks for waiting ^^