Print the parts. Clean them up as necessary. There is a cylinder that needs to be snapped out of the center of the hexagonal nut cavity. It supports the top of the cavity while printing.

I found it easiest to use the bolt to pull the nut into it's cavity as shown in one of the photos.

Screw the ball & bolt into whatever it is being installed on, place it on top of the socket and put a little weight on it. It should pop right in. If it's difficult, it might be that your printer is putting down a little too much plastic and either your printing profile needs to be adjusted or you can create a looser socket with the SCAD file.

I printed it in PLA. It may be too loose if printed with ABS but it can be adjusted in openscad.