This method for anchoring floss only works for even numbers of strands. Do not use when stitching with overdyed or variegated floss.
If you are using an even number of strands of floss for cross stitching, start with one half the strands (2 strands - use one, 4 strands, use 2) twice as long as you need. Fold them in half. Thread the needle so the ends of the floss are near the needle and the "loop" is the end farthest from the needle.