Singer Professional 5 Manual page 100

Thread Chainstitch Looper with a regular
overlock thread.
1. Raise the Presser Foot to release the
tension disks. Pass the thread from
the back to the front through the
thread guide
2. Thread the top cover thread guide by pulling the thread towards the left until
it slips under the guide. Then pull thread along the right side of the button
as illustrated.
3. While holding the thread firmly, pull the thread between the tension disks.
.
Triple Cover Hem page 6