Knitting in the round is a technique that is useful for a variety of projects, from socks to sweaters and headbands. You can knit in the round in a variety of ways; for a large project, you can generally use one pair of circular knitting needles. For smaller knitting projects, however, you either need double-pointed knitting needles (DPNs), two sets of circular needles, or one long circular needle for "magic loop" knitting. Any time you have a knitting pattern that calls for double-pointed needles, you can convert it for use with circular needles.
Cast on the number of required stitches that the pattern calls for. The only difference is that for a pattern using DPNs, it will tell you to divide the stitches between three or four needles. For magic loop, you will cast on all of those stitches on one needle.
Slide the stitches onto the cable portion of the circular needle. Divide the stitches in two, and separate the two sections slightly with your fingers. Pull the cable loop through the centre of the two sections of stitches, so now you have half of the stitches on one needle and half on the other.
Pull the right needle out of its stitches; slide the stitches back down the cable. Use the right needle to knit the stitches from the left needle. When you finish with those, slide the unworked stitches up onto the left needle and the worked stitches back down the cable. Continue to repeat this pattern around for continuous magic loop knitting.
Cast on the required number of stitches, using only one circular needle. Then transfer half of those stitches to the other circular needle by slipping them off the first needle and onto the second.
Slide the stitches down the cables to the other ends of the needles, lining up the cast-on edges in the centre. Keep the working end of the yarn on the right needle, and use the dangling half of the left needle to knit the stitches from the left needle.
Pull the right needle through the stitches to put the stitches on the other end of it, and turn your work so that this needle is now on the left. Use the dangling end of this needle to knit the stitches from it. Repeat this process around your work.
Remember that you are not changing the pattern, you are only changing the way you work the pattern. With DPNs you divide the stitches among three or four needles; with the other methods you divide the stitches among one or two needles.