This sketch of the 50mm f/1.2 shows how you can remove the entire lens from the barrel as a single unit. Usually it's easiest to do this first, and then do any further disassembly after the lens is out of the barrel. The front ring that has to be removed first is actually part of the focusing ring. Once it's out of the way, you can reach the set screw that keeps you from unscrewing the filter ring. After that, the whole lens just lifts out.
This sketch of the 50mm f/1.4 shows a further stage of disassembly. If you only need access to the front of the diaphragm, the front cell can be unscrewed from the lens without first removing the lens unit from the barrel.
The tool that I use to unscrew the front ring from the focusing grip is a piece of PVC pipe, with a ring of rubber glued to one end to make a sort of friction wrench. You will need different diameters for different lenses.