The turned brim begins with a provisional cast on. I use the crochet chain provisional cast on because, when done correctly, it is just so cool how the provisional cast on pulls out. Start with waste yarn (cotton works well) and crochet a chain at least thirty stitches longer then the number of stitches you need to cast on. How many stitches do you need to cast on? Swatch!
I am using a 100% merino fingering weight yarn. (Don’t ask me what brand; I lost that label a long time ago!) I cast on 40 stitches and knit for three inches. I measure my stitch gauge across the middle of the swatch. My gauge was 26.5 stitches in 4 inches or 6.6 stitches per inch. As I am going to donate this hat to the Hat Closet, I will knit for the average head, 22.5 inches. A hat needs negative ease to stay on the head, so I will multiply the stitches per inch by 20 inches (6.6*20=132).
Back to the chain, it needs to be at least 160 stitches long.