Bowwild- you take a large spool and run out a bunch of line to get the spool down to a smaller size- throw out the excess line. Now you have a smaller spool maybe a bit bigger in dia. (and a bit longer) than a sewing machine spool of thread. Take that spool and wrap it with a few wraps of duct tape. Now you have a spool with the ends open but the spool, as-a-whole, is wrapped so it can not come undone. Spools unravel from the center outward. So, on one end of the tape wrapped spool there should be a loose end of spool string (I just tuck that under the tape for storage).
Okay- now when ready to use the tracker, you just take that taped spool and roll it in your hands to loosen up the roll (it helps the thread pull out easier). Now attach that taped spool to your riser in any number of ways; tie it on, use velcro, or just tape it to the riser (that is what I do- I had friends use velcro).
Then take the loose thread from the center of the spool and tie it on your broadhead with a little slack (helps if the head is vented- otherwise you could do what Curt showed).
When you shoot- the thread will spool out until there is none left.
It just makes a simple and portable way to use a tracker. Like I said, I always carry a small spool set up (along with a 24-36" piece of tape)in my possibles bag/stuff. It takes up no space/wt., but in a mid-day downpour on stand it comes in handy.
Hope that helps
Dan