Circular knitting needles are traditionally used to knit things in the round, such as socks or sweater sleeves. They can also be used to knit flat items; the knitting just isn't joined in the middle. Circular needles come in various lengths and widths. The one to pick depends on the gauge and length of knitting you require. If the circular needle is too long, it will stretch out your gauge, so be sure to pick the correct length.
Pick up or cast on stitches to your circular knitting needles. This is your foundation for knitting. Do it the exact same way you would if you were using straight needles.
Arrange the stitches to prepare for joining. Lay the circular needles on the table and make sure the cast-on stitches are all on the inside of the circular shape.
Put a marker on the end of the knitting on the right needle. A ring or pin will do. This will mark the end of the round.
Pick up the needles, making sure the end of the round with the marker is in the right hand and the beginning of the round is in the left hand.
Knit or purl the first stitch from the right needle onto the left needle. Pull tightly to avoid leaving any holes.
Continue this same process around the needle until you come back to the marker. You will have to pull the stitches around the circle a bit to avoid pulling in the knit fabric you are creating.
Put the marker back on the right needle and continue another round.
Make as many rounds as needed for your knitting project.
Circular needles can sometimes be difficult to master, so it may be easier to learn the basics of knitting on straight needles before progressing to circular needles.