The best way, of course, to rethead a serger is to tie the thread to the existing ones.
Once in awhile, however, the threads will break and I have to do it from the beginning.
I had a horrible time with the last couple of steps on one of the threads, so -
I learned to poke a stiff thread 'from the top' when a thread would break through the 'arm' that had two holes in it - and connect it about halfway through to the thread that I had started threading from the spool. Then I would just pull that thread back towards the needle(s).
For the life of me, I could not get the serger thread to go through those two holes. Maybe beading wire would work, too?
Does this comment make sense to you?