Basically, there's three little screws in the mounting ring that have to be slightly undone (there is one that sticks out-leave that one be as it's for the mount, not for the retaining ring), then a retaining ring comes out. Before doing that, definitely spin the focus to as close as possible and take note of how far the aperture ring is from the focus ring-that will be important later. After the retaining ring comes out, the glass and aperture unscrew in one piece (it's sealed, so there's ZERO dust in there-MINTY fresh) and the focus ring and mount are left. I took a rag, soaked it in lighter fluid, put out the resulting fire, then cleaned out all the old sludge in the threads on both parts that was, at one point, lubrication, then re-lubricated it with some of my favorite gun lube (Breakfree CLP) and a Q-tip. It's a higher viscosity than most, but it's not grease, and it stood up to guns firing and being absolutely beaten on in all kinds of weather conditions without splattering all over when I had them, so I figured it was the right thing to use. Anyway, I put the focus ring to its closest setting, then repeatedly thread the aperture/glass assembly in (CAREFULLY to avoid stripping) until I got it right. After that, the retaining ring has a finger (for lack of a better word) that has to sit in the glass/aperture assembly to keep it aligned. There are also three little indents in the side of the ring for the screws. The trick is to line the the glass/aperture up, then drop the retaining ring on without pressing it in completely, take a moment to finely align the retaining ring, THEN press it in and screw it in place. After all of that, the focus is incredibly smooth.