Cast off/bind off with stretch?

I am “fairly” experienced with knitting, but have not done much for many years. I am working on a cute short sleeved sweater with puffed sleeves for a granddaughter. The directions say it’s essential to use a stretchy bind off BO method for the garment. Any help would be appreciated. Thanks.

One way is to use a needle two sizes larger than you knit the sweater with. You can also do this stretchy BO, or one where you k1, place the st on the L needle, k2tog, place st on left needle, k2tog, etc…