Look in manual follow their steps carefully, i had a hard time but finally got it done.
After you do it you'll say
This was not that hard. I found page 9 was the ticket
follow steps where it talks about installing pre wound reel. be sure to secure string in the little grab slots, once you insert back pull the 2 strings to unsnap them out of the grab slots to free them, that should work.
good luck.