Instructions
Save your game and turn your GBA off if you are currently playing, then connect the GBA Link cable from your GBA to a second one with a different copy of "Pokemon" in it. If you are not currently playing, connect both GBAs and then turn them on.
Walk into any Pokemon Center and go upstairs. Speak to the woman in the center stall to enter the Trading Room. Do this on both systems.
Go through your Pokemon and decide which one you want to trade. Scroll through the list of Pokemon and press "A" when you have decided. Repeat this process on the second GBA, but pick the Pokemon you want to trade to your party for the one you decided on earlier.
Repeat this process for all Pokemon that you want to unlock in your game. You can use any "Pokemon" game made for the GBA to complete your Pokedex and Pokemon collection. For example, you can trade Pokemon between "Pokemon FireRed" and "Pokemon Emerald" or ""Pokemon LeafGreen" and "Pokemon Sapphire."