It is going to take a few days to get a bit more of a hang of this. I did my first one in lead pencil, and then went over it with water colour pencils & blended with a water brush. Turned out to actually give the impression of a bit of a fold - especially if you squint your eyes a lot (and stand on your head!)
Then I used black pen and tried using scribbly lines to get the shadows. Not too successful.
For this one, I worked from both the real fabric and a photo of the fabric. I used a 6B lead pencil & put in the shadows first and then the stripes. I think I haven't looked at the shadow details carefully enough - not enough actual shadow areas.