We all know the best method of changing lenses safely and conveniently: 1. Find a nice comfortable chair. 2. Move it to a suitable horizontal surface (kitchen tables or desks do nicely). 3. Place the camera whose lens is to be changed on the surface. 4. Nearby, place the lens you wish to put on your camera. 5. Proceed to change lenses.
Now, that was simple, wasn't it? But unfortunately, if you're out photographing, you seldom have such luxurious appointments for changing lenses. At worst, you are juggling lenses, cameras, plus front and back lens caps and perhaps a sunshade and a filter or two while on the run. (I sometimes suspect that some photographers get married only to have two more hands available for emergency lens changing.) Dents and other physical lens disfigurements can often be traced to unsuccessful lens exchanges and the effects of gravity thereupon.
If you assume I am now about to reveal the steps for a perfect lens interchange system, forget it. No such thing exists unless you can grow a third hand. Lens change is and will remain inconvenient and a hazard for all, amateur and professional alike.
We are about ready now to try the best way I've discovered to minimize the bad bargain of lens change. First, admit to yourself that you cannot possibly manipulate a camera, two lenses, and two front and two rear lens caps all at once unless you are an accomplished juggler. The first step is to anchor at least one major component to some sort of safety device. That safety device is your neck. Despite the many times you have seen exasperated photographers holding a loose camera in one hand, a lens in another, and the second lens under an arm, it's a prelude to disaster. Instead, keep your camera's neckstrap where it belongs -- around your neck. That eliminates the need to hold the camera and keeps it safe from the dropsy syndrome. Hold the lens you wish to place on your camera in your left hand. With your right hand remove the lens' rear lens cap. If you have a shirt or jacket pocket, put the rear lens cap therein. No pocket? Hold it in your teeth. Don't just drop it into your camera bag or deep pants pocket. It will probably disappear among all the other detritus, as all lens caps are wont to do, and you may not find the darn thing again for hours or days.