Modified this to be circular, better match the Simply Slip hat, and to use up more of my clown barf yarn.

Cast on 84 with A and join in the round.
Row 1: purl
Row 2: knit
Row 3: purl

Then the slip stitch pattern is:
Row 4-6: With B, k3, sl1 around
Row 7: With A, k around
Row 8: With A, p3, k1 around

Repeat rows 4-8 until you reach the desired height, minus about an inch.
Then repeat rows 4-6 once.
Then with A:
Row 9: knit
Row 10: purl
Row 11: knit
Bind off purlwise.

As a tip, I did the p2, slip to needle, p2 tog, p1, slip to needle, p2 tog for bindoff, and it was too stretchy and makes it curl. I’d do a normal p2tog bindoff to eliminate the problem, I think.