Take a seam ripper to the entire crotch seam on an old pair of pants.
Just do it.
Once you’ve ripped out the entire seam, you can reach and maneuver to applique or patch or decorate however you want. I like to use Wonder Under and the method I describe here.
After you’ve embellished to your heart’s content, turn the pants inside out and sew the crotch seam back up. You’ll probably want to zig zag or serge to finish the edges because denim FRAYS.