You need some way for connecting your DS to the Internet via WiFi. This can be done with the official Nintendo Wifi USB adapter, or with any router with Wifi capabilities.
You use DLDI to patch homebrew so it works with a certain flash cart. However, most flashcarts nowadays have auto DLDI, so you probably don't need to worry about that too much.
you go to options and scroll down and there is options for wifi, something like connect and disconnect. you click on connect and then pick to seach for an access point