On the back of the hoop, use your needle to drag the "tail" of the floss under the loop formed by the back of the stitch. Gently tighten the floss to secure the end under the loop.
From Connie G. Barwick, former About.com Guide
On the back of the hoop, use your needle to drag the "tail" of the floss under the loop formed by the back of the stitch. Gently tighten the floss to secure the end under the loop.