Update: Turns out it's not possible to put the end-of-the-line blocker (large dark thing) at the very end of the line. When you look at the other side of the mag, there's really only one place it can go:
Once I've fed the spring in, the only thing to do is to hold tight and seal it up. I tried the follower on either side of the spring, but the spring is symmetrical and either end seems to have the same effect. I guess I'll try some lubrication next.