Remove the first cap that is leaking. In this picture, it is the fuel cap.

Using a flat blade screwdriver, gently lift the old seal out. In the case of the old black fuel seal pictured, it was very brittle; it had become about as flexible as plastic instead of rubber.

Install the new seal, stretching it around the two metal tabs that extend out of the center of the cap.

Repeat with the next cap, in this case the metal oil cap. That seal was still fairly soft, and was not leaking on this car, but was replaced as preventative maintenance.