At first I thought I'd assemble both parts and then affix them as a unit to the washer, but the knob kept popping off any time I pulled it out to use it. Then I realized that the pin in the timer cap acted to wedge the knob in place, so took the two piece apart, installed just the knob (which made it easier to see that it was aligned properly anyway), then installed the cap overtop the knob. The whole assembly was now secure and looks brand new again!