Ok, since I'm a very visual learner, can I make sure I understand this? The slip knot is your extra stitch. When you have cast on all you need plus the slip knot, you slide the knot off and then cut the yarn from the other source or outside of the ball and weave the extra in?
Is this kind of like doing a provisional cast on but not removing the extra cast on piece?
Sorry for being so dense but I seem to read this early in the morning when my brain is foggy and the caffeine hasn't taken effect yet.