Installing new threadless headset: how to set the crown ring and its rubber ring? My old headset was loosy and feeling very instable.

So I removed it, I got this: 
and I'd want to install it.

I am wondering what is the right way to install the two pieces on the right. I know the crown ring (second from right) has to be installed on the fork (I removed the old one). But I don't know what to do with the plastic ring. Should I put it before the crown ring? Should I put it afterwards? What is its purpose? 
(actually I am having and hard time realizing how everything works since the ball bearings are sealed and they don't have to slide on the crown ring itself).

I bought this PVC tube to install the ring, for reference:

And I think to hammer it down with it. 

Thanks for your help!
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