1. Allow a yarn tail three times the length of the intended cast-on edge plus a few inches extra for ease of working. Hold the yarn in a slingshot position as for a long-tail cast-on with the yarn tail over your thumb, and the working yarn over your index finger; use remaining fingers to hold and tension the yarn.
2. Place the needle on top of the yarn, between your thumb and finger, then twist it anticlockwise, coming up under the yarn; the yarn wraps around the needle, forming the first stitch. Alternatively, make a slip knot.
3. Take the needle from the front, under both strands of the loop around your thumb.
4. From this position, take the needle behind and over the strand on the far side of your thumb, then bring it back towards you under the nearside tail strand.
5. Bend your thumb down to open up the thumb loop, then pick up the yarn on your finger and pull it though the thumb loop.
6. Drop the loop from your thumb, and tighten up the new stitch. Pick up the tail yarn with your thumb again, and repeat from Step 3.