Start with a 4-6 foot length of yarn. I use whatever's on hand -- acrylic, cotton twine, handspun. Tightly spun, resistant to breakage is best. Not too thick, not too fine (no jute, no thread). Most of my leaders are about fingering weight.
Snug the half-hitch up tight, next to the first wrap around the bobbin. This is a very secure leader that won't rotate around the core as long as both wraps are snugged up tight. The cat's paw is AnnaBannana, helper extraordinaire.
If your leader should break, you can tie another simple overhand knot. If the knot is at the very end (usually it is), remove the leader from the bobbin and rotate the loop a bit before putting it back on the bobbin so the knot is not presented at the end. I change my leader after it breaks 3-4 times or if the twist makes it just look too unappetizing to spin from ... either of those usually takes at least a few months to occur.
If you do something different or have a leader tip to recommend, I'd love to hear it! Please comment on the blog or contact me. Thanks!