The first step is working out how much yarn to leave for the tail. Wrap the yarn around your needle once for every stitch needed, hold onto the yarn at that point and slide the loops off your needle. You may want to add a little bit more yarn to the tail to ensure you've definitely got enough.
Next you need to make a slip knot. Take your needle in your right hand and place your slip knot onto your needle, with the tail of yarn at the front. Pull on the tail to tighten up the slip knot. This counts as the first knit stitch.
You should have two strands of yarn coming from your needle, with the tail being at the front. Hold onto your needle with your right hand and put your left thumb and index finger between the two strands of yarn and grip the two strands together with your other fingers.
Part your thumb and index finger and bring them forwards, past the end of the needle, so you can see you now have two strands of yarn to work with, with the tip of your needle in the middle.
For a purl stitch bring the tip of your needle to the right over the yarn going to your index finger, under this strand and back to the middle.
Bring your needle over the strand of yarn going to your thumb and then downwards, bringing it across to the right beneath both strands of yarn, then upwards back to the middle, with a new loop now on your needle. That's one purl stitch complete.
For a knit stitch bring your needle to the left over the yarn going to your thumb, under this strand and back to the middle.
Bring your needle over the strand of yarn going to your index finger and then downwards, bringing it across to the left beneath both strands of yarn, then upwards back to the middle, with a new loop now on your needle. That's one knit stitch complete.
Repeat steps five to eight to cast on all the stitches you need.
We now need to complete two setup rows, these will be worked flat regardless of whether you're going to be working in the round. For the first setup row, knit one through the back loop, then slip one purlwise with yarn in front. Repeat that until you reach the end of the row.
For the second setup row, knit one, then slip one purlwise with yarn in front. Repeat that until you reach the end of the row.
You can now simply spread your stitches across your needle and join to start working in the round as you usually would.