How do you trade Pokemon on Pokemon LeafGreen?

Insurt the wireless adapter in the top of the GBA. Go to the top floor of any Pokemon center and enter the third room. From there you can trade with a friend any Pokemon you have in your party.
you can trade leaf green and fire red on the top floor of any Pokemon centre but to trade with sapphire ruby and emerald you will have to get the ruby and the sapphire (if you don't know what it is play the game and itll come clear)