How do I fit the rubber washer?

Our Shanks toilet cistern has an old brass pull-up plunger flush - the plunger is pulled up by a chain on the handle. It's a simple drop-valve, not a siphon. I think it dates from the 1950s but it could be older.

It needs a new rubber washer. The washer is about 60mm across, tapered into a slight cone and fits like a collar around a "Shanks Beta" disk with flanges in the plunger.

I have been able to track down the washer, but I need someone to tell me how to fit it. I am a complete novice at this, and would need an idiot guide. Can anyone help me.

For anyone looking for the rubber washer it is called a Dura or a Beta washer and I got mine from Graham the plumbers' mertchant