First, to figure out your gauge, knit a swatch about 5" x 5" and measure how many stitiches per inch you're getting. Then multiply it by the number of inches you need (the diameter of the hat). So if you're knitting 3 stitches per inch and you want a 20" hat, cast on 60 stitches. To follow the decreases in that particular pattern, you'd need to cast on a multiple of 9 stitches, so in this case, it would be 63.
Cast on your multiple-of-9 stitches, then follow the pattern as written, except I think you'd peobably want to decrease until 5-10 stitches remain, instead of 10-15, because of the weight of the yarn.