Need help with a pattern

Having issues figuring out part of a pattern I’m working on.
The pattern can be found here:

The part I’m having trouble understanding is this section:

[B]Shape Back:[/B]
Cut yarn leaving long tail. Thread tail through remaining sts and pull tightly to secure. Sew back seam for 2 (2½)" down from crown. Remainder is left open and becomes the lower edge of hat where the strap is worked.

[/B]With A, cast on 12(20) sts, pick up and k21(27) sts across left side base of hat, pick up and k1 st in seam, pick up and k21(27) sts across right side base of hat, cast on 12(20) st–67(95) sts.
Work 3 rows in Garter st.
Buttonhole Row 1: K2, yo, k2tog, k9(17), yo, k2tog, place marker, k to end.
Buttonhole Row 2: K to marker, yo, k2tog tbl, k9(17), yo, k2tog tbl, k2.
Work 3 rows in Garter st. Bind off.

I followed the pattern and got to the end of the “shape back” part. I threaded the yarn through the open stitches but haven’t sewn the back shut yet because I’m confused about the part with the strap and don’t want to mess it up.

Can anyone put this to me a little more simply? Still fairly new to knitting.

You’re going to be knitting an extended edging for the bottom of the hat that will also have the strap incorporated into it. If you look at the picture of the hat lying flat (without the cute baby) you can see the idea of the strap.
Take a new end of yarn and cast on the required number of sts for the size you’re making (this will be one half of the actual strap). Use a cable or knitted cast on here so it will be the same as the cast on for the other side (see Free Videos tab at the top, Cast ons for how to). Then pick up the given number of sts along the open part of the back (this is the edging). There’s a video to show you how to pick up sts under the Free Videos tab at the top of the page, Tips. Use that same cast on method as earlier to cast on sts for the other side of the strap. When you do this and look at your work, you’ll have the hat attached to the needle by the picked up sts with cast on sts on either side of it.
Before you begin this, you’ll need to pull the sts that are on the yarn tail tightly together and secure them and then sew up 2(2.5) inches of the back together.
God luck with this adorable hat!