Any hat knit in the round can be knit flat or on double points, but I think you want to knit flat. Look at your pattern, most hats knit in the round have a graph to follow, or tell you to knit every round. Add two stitches to the total number to cast on (these two stitches are selvedge stitches and should not be included in any stitch pattern being used, always slip the first stitch in each row as if to purl, and knit the last stitch in this case) and cast on flat, working back and forth. If the pattern in the round calls for you to knit all rounds, remember that stockinette stitch knit flat is knit the public side rows and purl the reverse side rows, so your alternate or even numbered rows should always be purled. Otherwise you can pretty much follow the pattern you plan to use as written. When you get to the end and have finished the shaping, leave at least 18 inches of tail yarn, run it through the last few stitches twice with a yarn needle, then pull these stitches up snug and, using mattress stitch, seam the edges together through the selvedge stitches and weave in your tail.