Learn the short tail cast on: one strand, no estimating how long the tail needs to be, quick and easy.
Step-by-Step written instructions with pictures:
Set-up: make a slip knot close to the end of your yarn (leave 3-4" of tail) and put it on your needle tip.
Step 1: Grab the yarn with three fingers (middle, ring, and pinkie) and wrap the strand around the outside of your extended thumb and index finger.
Step 2: Pick up the strand running from the needle to your thumb from underneath.
Step 3: Pass the needle tip over the strand running from thumb to index finger, then slide the needle tip up along your index finger to pick up a loop.
Step 4: Slide your index finger out of its loop.
Step 5: Pass the loop on your thumb over the needle tip.
Step 6: Drop the thumb loop and pull tight.
Cast on as many stitches as necessary, without having to estimate the length of yarn you will need. Because of the way this cast on twists the yarn, it is not recommended for single ply or loosely spun yarns, because they will come apart. Also be aware of how the stitches will "twirl" around your needle as you cast on, and be sure to un-twirl them before joining in the round.