Quoted from vid1900:You might have to cut the old, hard rubber off with a razor.
The new rubber is softer and will slip over the rivet with pressure.
What he said. ^^^^^ You can "lube" the new rubber before you press it over the rivet. Use a little water to wet the hole first. Pres directly at the center hole of the new rubber. Make sure your thumb seals the hole tight. The buildup of air pressure in there will force the hole to expand a little and this will make it a bit easier to go on.