How to knit a hat for yourself
Take a hat you've knit for yourself previously. Count the number of stitch repeats times that by the stitch count of each repeat, cast on that number. Knit for the length of your hand from heel to index tip. Start crown decreases, find out you don't have enough yarn. Curse, check your gauge, and rip out the hat.
Measure your head circumference, times this by your stitches per inch (You know, from checking your gauge). Take that number and times by .9 (a little negative ease helps the hat fit nicely). Cast on this number, knit for the length of your hand. Start crown decreases, find out your short one pattern repeat. Curse, try on the hat and figure it doesn't matter as it still fits nicely. Fudge the crown decreases to make it work. And there you have a hat! (Or at least it seems to be my process...)