Instructions
Talk with a friend who owns either "Pokemon Leafgreen" or "Pokemon Firered" to discuss trading Mankey. Your trading partner must have his National Pokedex before you are able to trade.
Link your Game Boy to your trading partner's with a connection cable (Sapphire is not compatible with the wireless transfer option). Visit your closest Pokemon center and go upstairs to speak with the representative in charge of trading. Your trading partner should do the same.
Select the Pokemon you want to trade for Mankey. Take away any items you don't wish to transfer and confirm the trade. The mankey will go directly into your inventory.