The 6212C is a Singer sewing machine model. The Singer Sewing Machine Company has been making sewing machines since 1851. A manual for this particular model, which is the same manual as the 6202, can be ordered from their website. A diagram for threading this machine can be seen on the Sew USA website. Threading it is not difficult provided it is done in the correct order.
Place a spool of thread on the horizontal thread pin holder at the top and back of machine and slide on the spool cap. Wind the bobbin with thread according to directions in the threading diagram.
With the presser foot raised, open the slide plate and insert the bobbin into the case,
pulling out about five inches of thread. Slip the thread through the slot and under the tension spring.
Replace the slide plate.
Take the needle bar to its highest position using the hand wheel.
Taking the thread from the spool to the left, pop it into the first thread guide beside the spool, then the second one located at the top and front of the machine.
Wrap the thread from right to left around the tension discs while holding onto the spool. Lift the thread up under the spring and to the right until it slips into the spring thread guard. Let go of the spool. Pull the thread through the upper thread take-up lever from right to left. Next, thread it through the lower thread guide, then through the guide on the needle bar. Thread the needle from front to back.
Pull out about five inches of thread to the left with your left hand. With your right hand, pull the hand wheel forward so that it lowers the needle then raises it, and pulls up the bobbin thread in the form of a loop. Straighten both threads and pull them gently to the left.
Threading a needle is made easier if the thread is first moistened and flattened.