how to long-tail cast on (ltco): slip knot, anchor, through, through, tighten

after making a slip knot,

anchor - use your left hand to separate the two ends of your yarn so it runs along the outside of your thumb and forefinger; grip the ends of the yarn between your remaining fingers and palm to create tension

through - bring the tip of the needle underneath the yarn on your thumb and through the loop

through - bring the tip of the needle on top of the yarn on your forefinger and through the loop

tighten - pull the two ends of the yarn to tighten your knot