Singer Quantumlock 14T957DC Detailed Threading Manual page 39

Thread the eye of the lower looper as shown, making sure that the thread is
positioned in the V cut out in front of the looper threader. Pull the thread tail to
the back of the serger approximately 4 to 6 inches.
Tugging slightly on the thread tail at the back of the serger, push threader lever
all the way up and to the left to hook the thread into the eye at the back of the
looper.