You have 56sts on the needle. Put markers on either side of the center 16sts. The needle will look like this: 20|16|20.
Start the row by binding off the first 4sts. That means, k2, lift stitch one over stitch 2 and off the right needle. Continue like that until you’ve dropped 4sts over and off.
Now knit up to the first marker. Drop the marker, k2 and lift stitch one over stitch 2 and off. Continue like that until you’ve dropped 16sts off the needle. Knit to the end of the row. The needle will look like this: 20–16 bound off sts–16sts–4bound off sts.
Turn to the WS and bind off 4sts, purl to the center neck gap, drop the yarn strand, pick up the strand attached to the other shoulder and bind off 5sts at the neck edge, purl to the end of the row.
Turn to the RS, bind off 3sts (this is the armhole edge), knit to the neck gap, drop the strand of yarn, pick up the yarn attached to the other shoulder and bind off 5sts at the neck edge and knit to the end.
Repeat like the above binding off 3sts when you start an armhole edge and 5 sts when you start a neck edge until you’re left with one stitch on the needle on each side, cut the yarn leaving about a 6" tail to weave in later. Pull the yarn through the last stitch.