Line up the front side of the head (where the pattern selector is) with the front side of the mic amp (front side circuit board says "U89"). Now insert the head with its pins, starting from the back, where the thick brass guide pin is situated, into the corresponding holes in the mic amp, AND and the same time, press the two spring-loaded side bars of the mic amp inwards at the top, so their nipples can slide underneath the head basket and lock in as you push down the head. If successful, you will hear a click, and you will no longer be able to pull off the head.
This whole procedure is a bit tentative for the novice, because newer U87Ai use contacts that are not as smooth during insertion of the pins as they used to be. Just be gentle, and the pins will fit into the contacts eventually.
Once this is done, slide the housing tube over the amp, as you keep an eye on the guide nipple situated inside and front (Neumann badge) of the tube. The nipple needs to slide into the cutout in the connector bell of the mic amp. Wiggle the housing tube a little bit back and forth as you slide it upwards. Finally, screw the bottom ring back onto the connector thread- just hand tight- don't crank it!