Shoulders like that aren't to hard to do. The hardest part is probably to get the
GS evenly on the shoulder.
Fix he shoulder to something. The model itself or glue/blue
tac it temporarily to something.
Apply a bit of
GS on a regular shoulder and fill in the area inside the raised edges.
Use your tools and a wet finger to get it smooth.
Use a tool with a sharp edge (a hobby knife for example.) Press the random pattern in piece by piece with a moist tool. Avoid pulling and try to press instead to avoid ripples in the
GS surface. Just make sure with a bit of water (or saliva, bvaseline, oil, ...) that yout rool doesn't stick to the
GS if you remove the tool from the pressed line.
You can see two shoulders i did like that on the two guys on the right.

Repeat until all
GS is covered in the pattern.