I always take off the top body Otherwise it's very difficult to align the top of the pin to the middle shell. You have to see the metal part of the pin in the hole before you press the pin into place otherwise you might bend it.
I have to use 2 pairs of pliers to tension the pin before I push it all the way in. You can't really push it in until it's tensioned a little otherwise the end of the pin will scrape on the plastic middle shell. That's where my blood usually starts to flow. When I'm using 2 sets of pliers and my finger always gets pinched in one of them.
Once you have put tension on the spring and have cleared the sides of the middle frame with the ends of the pin then you can press the pin down into place with a Flathead screwdriver. Don't push the pin all the way in. Push it most of the way in but then flip the drone over and make sure that the top of the pin is going into the hole on the top of the frame. You can then push the rest of the pin and. I use a vise grips with one end on the bottom of the pin and the other end on the top of the middle frame.