What I do is put the spring and plunger back in first. Then I take the pin and start it from the top of the coupler and about halfway through the center gap. Orient the spring properly, the long tab inside and the small bend facing down and slip it on the pin. Push the pin through to capture the spring. Now, take the knuckle and slide the spring up to the top, set the knuckle on the pin under it. Slowly pull the pin so that the knuckle drops into it's slot but you still have the spring on the pin. Make sure you have the knuckle latching surfaces inside the coupler when you do this. Push the pin through the knuckle and then you can clinch the rivet. It's harder to describe than to do, once you've done a few it's pretty easy.
While you have the knuckle out, you can smooth the latching surfaces of it, makes the coupler operate much more smoothly.