The challenge here was to take a relatively simple surface pattern and use that to reflect the 3D nature of the object.
Looking at my stripy pyjamas this morning I took a couple of photos of the folds:
The first drawing was done in watercolour pencil -
Didn't work out too well - the valleys and peaks ended up in reverse I thought...
I used a fine marker pen for the second drawing below to see if varying the thickness of the pattern lines a bit more carefully would give me better results. Also some smudging done with water for depth and shadows-
I then decided to look at different fabric, a check instead of only stripes -
Again I used fine pen and a little watercolour pencil smudged with damp paintbrush for shadows.