This will take patience, time, skill, and jolt cola. Tim Schuerewegen released the driver and application necessary to allow anyone with a Nintendo DS, a GBA flash cart, and a supported wireless network card to load any homebrew demo/app on their DS. You could do it with PassMe
but now it can be done wirelessly.

What You Need
PCI or PCMCIA wireless network card with the RT2500 or RT2560 chipset
(complete list here)
NOTE: USB version (RT2570) will not work
* Tim’s modified driver and application (local mirror: driver, application)
* Gameboy Advance flashcart (more info here)
* A computer with Windows 2000 or Windows XP

just take your time and read everything step by step