When it comes to knitting in the round, most people either love it or hate it. No matter which camp you're in, it's likely that you will, at some point in your knitting career, fall in love with a pattern that is designed for the other side. If you are like me, you will probably wonder why in the world the designer would write this to be a flat/circular pattern when it so clearly works as a circular/flat pattern. You may even use some strong language, depending on how much you really, really need to knit this pattern.
But wait! Put away the strong language and get out your pencils. You can convert many patterns in either direction (I'll talk about exceptions in a minute). And guess what? It's really, really easy. The one thing you need to remember is:
When you are knitting flat, you are alternating working on the "right" side and the "wrong" side. When you are knitting in the round, you are only working on the "right" side.
What this means practically speaking is that when you are converting patterns in either direction, you will need to reverse the stitches on every other row (I said it was easy; I didn't say there was no work at all involved). Let's do an easy example, stockinette stitch:
Flat R1: Knit
Flat R2: Purl
Circular R1: Knit
Circular R2: Knit
When it gets tricky is when you are working on something a little more complicated, like a cable pattern. Just remember that all you're doing is reversing knits and purls on every other row:
Flat Rows 1, 5, 9, 13, 17 and 21 (RS): P3, *K8, p3, k8, p3; rep from * to end.
Flat Row 2 and all WS rows: *K3, p8, k3, p8; rep from * to last 3 sts, k3.
Flat Rows 3, 7 and 11: P3, *[4-st RC] twice, p3, k8, p3; rep from * to end.
Flat Rows 15, 19 and 23: P3, *K8, p3, [4-st RC] twice, p3; rep from * to end. Row 24 Rep row 2.
Circular Rows 1, 5, 9, 13, 17 and 21 (RS): P3, *K8, p3, k8, p3; rep from * to end.
Circular Row 2 and all WS rows: *p3, k8, p3, k8; rep from * to last 3 sts, p3.
Circular Rows 3, 7 and 11: P3, *[4-st RC] twice, p3, k8, p3; rep from * to end.
Circular Rows 15, 19 and 23: P3, *K8, p3, [4-st RC] twice, p3; rep from * to end. Row 24 Rep row 2.
Now, there are patterns that should not be converted. Obviously, any pattern that has a flat finished project, like an afghan, shouldn't be joined and knit circularly. I would also recommend staying away from converting patterns from circular to flat that have cables or twists on both odd and even rows. Finally, any particularly large or heavy sweater should be knit flat and seamed -- the seams add stability and will help the sweater maintain its shape.
With those few exceptions, you can rework just about any pattern to be knitted in your preferred style. Try something simple first, like a knit hat. Soon you'll be on your way to converting every pattern you see, and enjoying the freedom that comes with using the style you prefer to create your knitted projects!