Thumb Gusset Help please!

I’m new on this board… I’ve been knitting for about a year now and I need a new challenge, so I’d like to attempt a pair of mittens. I made a sock (with the assistance of my wonderful grandma) last year, so I can follow a pattern for the mitten, but I have no idea how to make the thumb. Can anyone out there help me? The videos on this site are amazing, but I can’t find one that shows me how to make the thumb.
Thanks in advance!

Try here. Just scroll down past the ad.,2025,DIY_14141_3053226,00.html