There are many different ways to cast on. I use one of the very simplest, and it hasn’t failed me yet.
Take a circular needle, and a ball of yarn. Work one end of the yarn free, enough to give you some slack. Take one end of the yarn and tie a simple knot, and slip it loosely over one point of the circular needle. (This will be the needle that you hold in your left hand – or it is for me, being right-handed.)
With the other point of the circular needle in your right hand, knit a regular plain stitch through the loop of the knot on the left needle, and then slip it onto the left needle alongside the loop of the knot.
Now, using the tip of your needle which you’re holding in your right hand, pass it in between the two stitches on the left needle point to emerge on the other side, loop your yarn over it, and draw it back towards you, pulling the yarn with it through to this side. (In effect you are knitting a stitch.)
You will now have the 2 stitches you have already created on your left needle point (or one knot and one stitch, to be accurate), and the stitch you have just made on your right needle point.
Slip the new stitch onto the left needle point to sit alongside the others. You now have 3 stitches.
Repeat for as many stitches as you require, always slipping your right needle in between the most recent 2 stitches you have created when you create a new stitch.
(Incidentally, if you’re anything like me you’ll make stitches of all sorts of shapes and sizes. Some will be big and lumpy, others will be tiny. Don’t worry – it doesn’t matter in the slightest. It just looks untidy because that’s all you’ve done so far – but trust me, once you’ve knit a few inches of ribbing your cast-on row won’t be noticeable at all. The important thing is just to get them done.)
I keep track of how many stitches I’ve cast on by using stitch markers. After every 50 stitches I tie a loop of differently-coloured yarn onto my needle, so I always know how many stitches I’ve done.) I lose count easily, and there are few things more frustrating than losing your way after several hundred stitches (“325…326…327… Wait – was that 327 or 328? Or 329? How many? Oh bugger…”)
Marking off every 50 stitches gives you an at-a-glance reference, and takes all the stress out of counting. Afterwards, you just slip the loop markers off on the next row as you come to them, and if you keep a couple to one side you can use them to mark the seam stitches on the body. (Waste not, want not…)
Now, as you cast on the stitches they’ll start to bunch up and twist, corkscrewing round on themselves to an extent that you’ll wonder if you can ever get them all the right side up again. I’ve found that there’s nothing much you can do about this. The best thing to do is not to worry about it, just let it happen, and wait till you’ve got all your stitches cast on; then you just have to secure one end and patiently pass the stitches through your fingers as though you were telling rosary beads, straightening them out as you go.
The stitches will resist – they really won’t want to be straight, and sometimes you have to repeat it a couple of times just to be sure you’ve got them all aligned properly. But you really have to do this, or else your knitting will have a twist in it and that would be a Very Bad Thing – you’ll end up with a figure-8 Moebius-strip gansey instead of a nice tube.
When you’ve got your cast-on stitches straight, all you have to do is carefully bring the two ends together, and you can start knitting in the round as usual.
Remember, if you’re knitting a ribbed welt of knit 2/purl 2 ribs, you should cast on 10% fewer stitches than you need for the body. So if your body is 400 stitches, you should only cast on 360 at this stage. (You increase the rest after the ribbing.) But, of course, if you are knitting a ribbed welt, it is also vitally important to remember to cast on a number of stitches that is divisible by 4 (for however many knit – knit – purl – purl ribs you need).