Casting on with your thumb is a fast way of working a long-tail cast-on, perfect for yarn with a firm twist. This method follows the knitted cast-on method, but uses a thumb instead of one needle. It's faster than working with two needles, hence its popularity!
How to cast on with your thumb step-by-step
Pull three times the width of the project you are knitting.
Wind the yarn around your thumb, and insert a needle knitwise.
Bring the yarn around the needle.
Bring the needle through the stitch and on to the needle. Pull to tighten.
Repeat. And you're finished - beautifully done!