here's what I would do... maybe not what everyone would do.
To make sure that your ribbing all works out well, you need to cast on a multiple of 4 (88 st instead of 90).
every round for 4 inches, you're going to K2, P2
here's where it gets a little harder because you want the decreases to be even around the hat. 88 is a multiple of 8 so we'll use that as our multiple.
for the broken rib pattern, you will
round 1: k1, p1
round 2: K
round 3: k1, p1
round 4: k6, k2tog repeat to end
odd rounds: repeat round 1
round 6: k5, k2tog
round 8: k4, k2tog
round 10: k3, k2tog
round 12: k2, k2tog
round 14: k1, k2tog
round 15: k2tog repeat across, you will have one stitch left. k2tog this with the next stitch on the needle.
this will leave you with 4 stitches. make your I-cord and you're done.