Ah yes, the infamous "I wore this dress three times, now look at it" effect!

Here is the quick version - and I admit I had to pull up the old scene files to remind myself how it works: In any of my recent procedural fabrics you will find a "Thread Variations" node with, among other things, a "min" and a "max" setting. Dial the max up to some value between 1 and 2, depending on how thin you want the tattered bits, then feed in a node that creates a mask for it. I'm using a Clouds node here, like so:

Fiddle with the "max" and the parameters of the cloud node until you get the desired effect and remember that the brights bits are where the fabric will be thinned out. If you have the patience, you can also hand-draw a mask in order to place the thinned-out bits exactly where you want them.

-- I'm not mad at you, just Westphalian.