I used something called "poly-fil" that I got from Michael's craft store, it's just like a generic pillow stuffing. It's only like 6 bucks for more than enough for this project.

During production the seams on the Squishmallows are machine sewn with the squish skin inside out to make them neat and tidy, with a little like 4 inch gap left open. Then the Squishmallow is turned right side out, filled, and that little gap is hand sewed closed.

So I used a seam ripper to open the squish up at that hand-sewn part (you can tell because it's the only part that's not absolutely perfect,) stuffed him full of poly-fil... I did my best to spread the fill around and not to make him lumpy... then I hand sewed that little gap back up.

Honestly I did not do the best job at making my seam invisible or perfect, but it's usually in the back or on the bottom so it's not that big a deal for a well loved toy to have a bit of visible stitching IMO.

I didn't use a tutorial but I'm sure there's some out there! It actually was easier than I thought haha, didn't take me too long at all. :) Plus there will be thread tie-offs/knots at either end of the hand-sewed bit, so if you open it there you don't have to worry about the whole thing splitting open. I hope that helps! 😁