To wind the bobbin, place the spool of thread on the spool pin, thread through the tension discs, and start the machine while holding the thread end.
The SINGER 338 can accept two needles although these are usually the same size.
Make sure the needle thread tension and the bobbin thread tension are on the proper settings, and that the needle and the thread used for the fabric is correct.
Use the lint brush to remove accumulated dust and dirt and, as the company manual directs, lubricate the machine after each cleaning.