Many popular ROM hacks, such as Radical Red or Infinite Fusion, have this feature built directly into the game code. In these versions, every battle naturally yields double the experience points without the player needing to do anything extra. This is often the cleanest way to play, as it requires no external tools during gameplay.
There are two primary ways players implement this multiplier into their FireRed experience: pokemon fire red exp multiplier x2
If a true multiplier feels too glitchy, use the Wild Pokémon Modifier to battle high-level Chanseys. Combine this with the Lucky Egg item (found on wild Chansey in the Safari Zone) to achieve an effective 2.5x natural EXP. Many popular ROM hacks, such as Radical Red