When rethreading your serger leve it threaded, just cut the thread at the spool, put your new thread on. Take the old and new thread and tie a knot now cut off any tail of long thread (do all four at once) run the serger and the thread will go all the way through , you will have to thread the needles the knots wont go through the eye.
I have a Singer, (Four thread) never had a problem with it .