1. Stop driving.
2. Turn off the engine.
3. Let it cool down a bit and open the hood (not the Thunderbirds character).
4. Oh, and disconnect the battery.
5. Note the positions of the HT leads on the distributor cap. Remove them.
6. Release the clips wither side of the distributor cap and lift the cap off.
7. Tear up your note of the positions of the HT leads.
8. Replace the distributor cap if it is in good condition, or fit a new one.
9. Say "Oh no! I have no idea what order these HT leads should go in, and if I get it wrong I could wreck the motor."