It's super simple to do all you do is switch your stitches. Your last stitch becomes your first stitch and your first stitch becomes your last stitch. Below you can see just how easy this great little technique is.
Here are some stitches that have been cast on for a hat.
Slip the first and the last stitch off the needles.
Slip the the first stitch onto the right hand needle.
Slip the last stitch onto the left hand needle.
Place your marker and you're ready to start knitting in the round.
Here's what the join looks like after a round of knits.
See the easiest little trick for closing up that gap. This trick is best for any join you have in the round. I like it for after thought heels and thumbs or fingers on mittens and gloves.