1. Start with a slip knot on the left needle (or see below for no slip knot). Insert the other needle as if to knit, wrap the yarn and pull it through.
2. Give the new stitch a quarter turn and then place it on the left needle.
3. Insert the yarn betwen the 2 stitches to create the next stitch.
4. Quarter turn the stitch and place it on the left needle.
5. Repeat the cast on between the last two stitches (steps 3 and 4) until you have the number of stitches you require.
As you work the stitches use your thumb and forefinger on the left hand to keep them even at the base (don't pull them tight to the needle).
Loosely cast on stitches:
Working without a slip knot: for some projects I find it better to not use a slip knot - I just pull the yarn though once (instead of twice) or just cross it at the base and then cast on the regular way.
I secure the end as I weave in the yarn end.
The advantages are that if you have too many stitches, it's easy to undo a few and also if you are making a semi circular shawl you can pull the center stitches a bit tighter.