Singer Quantumlock 14T957DC Detailed Threading Manual page 21

Slip the thread into the third slot closest to the back and then up and over the
take-up lever slot.
Slip the thread into the third slot on the right next to the take-up lever, then into
the third slot on the right in the guides just above the needle clamp. Next slip
the thread behind the metal guide and then into the eye of the needle in (B)
position, and then under what looks like a bridge at the back of the foot. Pull
the thread tail to the back of the serger.