HOWTO: Make folds/creases at line edges ?

[Blighty]

1)
Create the T shirt layer. The T shirt must have a transparent background.
https://imgur.com/a/hi9hinS

2)
Create the Bump Layer. I used the crumpled image from your post #3
Note 1: This layer is best done in black, white and grays only
Note 2: This layer must be the same size as the T shirt layer. Rotate, scale, crop as required.
https://imgur.com/a/hPSLQuI

3)
Make the Bump layer invisible by turning off the eye next to layer
In the Layers Dialogue select the T shirt layer
Filters > Map > Bump Map
Use the Big Question Mark to select the Bump Layer
Set the Depth as required
OK
https://imgur.com/a/fbo386C