How to shirr?

I am knitting a child’s hat that ties under the chin. It is about 15-18 stitches across and knitted 12 inches in length. I have used worsted weight yarn.
The directions say to leave an 8 inch tail of yarn after casting off. I am then to use that tail to “shirr” the sqaure end of the hat, making it rounded. I am not sure what needs to be done. Any help would be greatly appreciated. Goober383.

Are there eyelets to pull the yarn through? If not, maybe they want you to weave the end in and out across the edge so you can gather it to be tighter.