Very helpful article, and if I may be so bold, I'd like to add some detail that will greatly increase odds of a successful install, and not end up with a damaged retainer pin stuck in the bolt head...don't ask me how I know.

To start with, get your hands on a 1/16" pin punch or drill bit, and a cartridge case. Note that the pictured bolt head is a right eject style, for a left eject you'll be doing the opposite that I describe.

In general, you want to install the retainer pin starting on the side of the bolt that the ejector hole is located. On the pictured bolt head, this would be on the side with the machined flat however, on a left ejecting head, the flat is on the same side as the picture, but the retainer pin would be installed opposite the flat.

Assemble the ejector and spring as shown. Snap a cartridge case into the head and pull it straight, compressing the spring and ejector. I am right handed so I hold the bolt in my left hand and use my index finger to hold the cartridge against the spring.
Slide the 1/16 punch or drill bit into the retainer hole starting on the side opposite the flat. Push it through to the opposite side. It should almost, but not quite protrude from the head. You can remove the cartridge case at his point as it's job is done.
Now slide the retainer pin into the head from the flat side as shown in the picture, and tap it in using a suitable driver such as a small hammer, driving the drill bit or pin punch out the opposite side as you go.
Again, if working on a left ejecting head, work from the opposite side as in the picture.

This method will insure that the ejector is deep enough and is not cocked in the bolt head, causing the retainer pin to become damaged while tapping in. It'll also keep parts from flying all over the shop. Don't ask....