I continued with hand practise today. I have discovered one or two things. I tried to use a Green Stuff and Milliput mix, but it’s a bit too brittle to carve down with a knife. I also found this video that shows a similar technique, with the main difference is that he uses much thinner GS fingers and then building the clay up in layers. I found this to be a bit easier for me, as you can see from the first two pictures, especially when I swapped to a smaller brush for the clay.
Thanks for looking