Instructions
Plug your GameBoy Advance wireless adapter into the cartridge slot on the back of your console. Insert the wireless adapter-compatible game into the cartridge slot on the adapter. If your game is wireless compatible, it will say so in the "features" section of its included manual.
Turn on your GameBoy Advance. This will turn on the wireless adapter, which will load the game.
Select the "Multiplayer" menu screen.
Select "Scan Mode," "Direct Connect" or whatever similarly worded option is available in your game's menu. This will allow you to see a list of other people playing the same game who are also using a wireless adapter in your immediate area. You will then begin a mulitplayer game with those people.