For my Undone Capelet, I made a video to demonstrate how to perform a one-row self-reinforcing buttonhole.

The video works row 5 of the pattern, which is:

Row 5 (RS): YO, K2tog, K1, BH, K2tog, YKY, SSK,
P1, K1, P2.

So it’ll also help if you’re unsure how to work a YO at the beginning of a row.