This technique comes from Elizabeth Zimmermann’s Knitter’s Almanac (Dover, 1981).
Make a simple loop of yarn with the short end hanging down (Figure 1). With a crochet hook, *draw a loop through main loop, then draw another loop through this loop (Figure 2). Repeat from * for each stitch to be cast on (Figure 3). After several inches have been worked, pull on the short end (shown by arrow) to tighten the loop and close the circle.