How to focus a Nikon 50mm f/1.2 Ai-S (at f/1.2) on Nikon DF?

The only thing I can think to add to the answers already given, would be to use a focusing rail if the focus throw on the lens is not too great, such as 90 degrees(1/4 turn) or less. With an old manual focus lens like that, I would hope the focus throw would be in the area of 360 degrees or more.

You can look at the image itself and see it if it blurry or sharp instead of relying on the arrows. Further, unless aspherical elements are used, and it is almost certain that they were not used on the old lenses, a fast lens like this will not be very sharp at maximum aperture. Even an f/1.4 lens would not be sharp at f/1.4 because of spherical aberration. Light rays from the edges of the lens will be focused at a different point thN light rays coming in through the center of the lens. That is why it is called spherical aberration.  Depth of field is also very shallow at such wide apertures, so focusing accuracy is paramount.

It's time to use live view which allows you to do critical manual focusing. You can magnify the live view image quite a bit and center the view on the object you want to focus on. This requires using a tripod. Shooting with apertures wider than f/2.8 takes care.

Possibly try focussing visually, rather than the green dot.

Note that the depth of field is very small at f1.2 or f1.4 thus the slightest movement of your body or your sitter's body after focussing can throw the focus off..

Note that the aperture gets smaller as the aperture number gets larger, thus the opening at f8 is much smaller than the opening at f2.  It is wrong to speak of a aperture of f11 as a 'large' aperture.