1. Travel to Mt. Moon: Make your way to Mt. Moon, which is located between Cerulean City and Pewter City.
2. Enter Mt. Moon: Once you reach Mt. Moon, head inside the cave.
3. Locate Clefairy: Inside the cave, you'll encounter wild Pokémon. One of the Pokémon you can find in this area is Clefairy.
4. Wild Encounter: Encounter a wild Clefairy and initiate a battle. Use moves and status conditions that will not faint the Clefairy. Try to weaken it enough so that you can capture it.
5. Capture the Clefairy: Use Poké Balls to try and catch the Clefairy. You may need to weaken it further or put it to sleep to increase your chances of capturing it successfully.
6. Return to Pallet Town: Once you've caught Clefairy, make your way back to Pallet Town.
7. Visit Professor Oak's Lab: Head to Professor Oak's lab in Pallet Town.
8. Talk to Professor Oak: Inside the lab, talk to Professor Oak. Show him the Clefairy that you've captured.
9. Starter Evolution: Professor Oak will express surprise and excitement upon seeing the Clefairy. As a reward, he'll offer you a choice of one of the three Kanto starter Pokémon: Bulbasaur, Charmander, or Squirtle.
By following these steps, you'll be able to catch a second starter Pokémon in Pokémon FireRed. Remember that you can only choose one of the three starters offered by Professor Oak, so choose wisely!