Singer QuantumLock User Manual page 12

10. Thread the looper eye (A) from front to back. Pull about 4 inches of thread.
Position thread into V cutout (B) on threader. Firmly hold thread tail with left
hand and pull Threader Lever (C) up into place (D).
"V" cutout
NOTE: Make sure the Upper Looper thread is
behind the Lower Looper thread.
Three-Thread Overlock (narrow) page 11
Threader Lever