I've taken the top of the machine and the side panel off. Then unscrew the plastic screw in the center of the knob. Work the outside belt off so you can remove the knob.
Here's what the knob looks like inside. There are 2 black plastic knob parts, one black part with an oval hole, one white part and a small spring.
The first thing to do is to put the spring in place. It just sits in the little trough that is molded to just the right size.
Next, put the white plastic part in. The white part has a post on the underside that fits into a small round hole.
Then put the black part with the oval hole onto the center shaft. The protrusion will go into the square hole in the handwheel.
Then put the center knob on top, with the black post going into the curved trough on the knob.
Put the washer on the plastic screw, and carefully screw it into the shaft. Don't tighten it too tight, or you'll break the screw off. Dang! It's a good thing I can get a replacement part #0401655-BSCREW Pfaff Hand Wheel Black Plastic. Retail on those is about $6.