you need:
jeans
narrow cutting board or cardboard piece
knife
vacuum cleaner

Slide the cutting board into the leg – this is to avoid the back side being slashed.

It’s best if you wore the jeans to see where to start slashing. I started just above the knee.
Start slashing the jeans horizontally – Do them in sections instead of the whole width of the jean for more destroyed effect.

Slash first, then scrape off the denim to reveal the threads.

Pull out the denim fluff and discard. This will cause considerate amount of dust, so have a vacuum cleaner ready.

Result:

by park&cube