Note that these things have nothing to do with internal functionality of the focus or aperture, nor the condition of the glass. That should all be good before even thinking about this. No sense making lens ergonomics better if the lens isn't known to be worth using!
Step 1: Prepare The Lens
Now clean the lens body. Wet cleaning works best, but remember that "wet" means ever so slightly damp. No liquid should be able to get inside the lens. Soapy water, alcohol, and very sparing use of other solvents can all work on metal-bodied lenses, using either paper towels or cotton swabs. A little alcohol on a cotton swab is usually best for cleaning details like engraved lettering.
Step 2: Reduce Visibility Of Wear Marks
If there are unwanted deep nicks and engravings (such as scratched-in ID numbers), they can be colored by this method but not filled-in. If you want to try to fill-in the marks, wait until the next step and then try using several passes of black acrylic paint.