Ed...it is not too difficult once you get the hang of it. The problem is that it is much easier show someone how than it is to describe the process. As you surmised, they install from outside. They squeeze between the gasket and the frame. The trick is to get them there without pushing them on through. It can be frustrating at times, but when you get them in, they stay in pretty well until somebody kicks them in carelessly. You can close the ports when they are in but it is hard on the clamps.
I start by sticking the bottom with the two tabs in place. Then I gently push the edge of the screen in, moving clockwise, and starting at 7:00. By the time you get to 3:00, the screen is bowed and when you get between 4:00 and 5:00 you have to push it the rest of the way in very gently, but firmly, and hope you don't go too far (sheesh this would sound disgraceful out of context).