The Singer 99 takes a regular class 66 bobbin.
To wind the bobbin on the older 99's you take the thread from the spool to the wire hook on top left, then to a little notched hook on the bottom of the bobbin winder, then to the arm that swings left to right across the bobbin as it winds, then to the bobbin. I wind some thread on the bobbin by hand a few turns before putting the bobbin on the bobbin winder. The bobbin has a little notch and there is a tiny tooth on the winder that fits into that notch. Then push down the clamp to tighten the winder against the wheel. Turn the flat knob on the inside of the handwheel counterclockwise to disengage the machine. --I can send a picture if you want one.