Once you figure out your gauge, you can just use this pattern.
Do a gauge swatch, and let's jus tfor the sake of example, say that your head measures 20 inches. If your swatch is knit at, say, three stitches per inch, cast on 60 stitches. The hat pattern is written for a multiple of 6 stitches, so it will just need a little tweaking.
When you start your decrease rounds, you'd start with k9, k2tog and work your way down, until 30 (or 24?) stitches remain, then start doing the decreases every round.