Installing new threadless headset: how to set the crown ring and its rubber ring?
The rubber ring would most likely be a protective seal of some kind, although for which part I am not so sure. I changed my headset earlier last year and don't remember such a part, although my memory isn't great.

Can you not determine the possible location based on the rubber ring's size? Most forks are tapered and are larger at the bottom, so the size might make it self-explanatory. My educated guess is that it will be an additional seal which sits between the cartridge bearing/compression ring (the brass coloured ring on the far right) and the dust cover.

I terms of installation, the second to right part (the crown race) should be fitted first onto your fork first. Ensure that the surface is clean and no corrosion or other dirt is on your fork from the old race as it won't sit properly. It's unlikely that this rubber ring would go underneath the crown race or above as it would prevent the bearings fitting correctly (the space is usually extremely tight to start with).

Although the following image doesn't show the rubber ring, it gives a good indication to the installation order of the rest of the parts. I suggest that the rubber ring goes between the 'dust cover' and 'compression ring' on that photo.

An exploded photograph of a bicycle headset, showing the parts in installation order