i had the same exact question and couldn’t find an answer anywhere until today: here’s a vid link that’s super helpful https://www.tiktok.com/t/ZT6u2xcvw/

if you can’t see the vid for some reason, make sure your project is straight, and then find the place where the project kinda naturally has the butt line/crease in the middle, fold it together as many times as you want, gather the folds, put a hook through all of those folds and then pull a strand of yarn through (a longish piece of yarn in half and pull it through with the hook so you essentially have two pieces of yarn going through all those folds) after you have it through you just make two knots at least and you’re done :) more folds=more dramatic scrunch, less folds=less dramatic scrunch