Fairly easily done! Using a #12 or #14 crochet hook (really, really tiny!!), put the ring on the hook, catch up the loops of the leader, slip the ring down on the threads, and with the loops still on the hook, catch up the loop on the butt end of the leader, and slip it through the loop on the crochet hook. Keep pulling the butt end through till the tippet end is at hand, hold the ring, and form a locking loop behind it. Pull it down tight, and it's not going anywhere!