Part 1- Set up your PC

Download devkitProUpdater-1.5.0.exe.
In order to test Nintendo DS programs on PC- we need an emulator. You can download it from here.

You can skip Part #2 if you are using an emulator.

Part 2- To Set up a Nintendo DS console

Download the firmware from here so that programs can be executed in the Nintendo DS hardware. Our card is marked v1.4I - make sure you download the correct version.
Extract the firmware.
Copy the entire files (all the files under R4i.V1.34b English ) into a Micro SD card using a USB adapter, which is from a R4i (red) box.
Put the Micro SD card into the nintendo DS adopter. Put the nintendo DS adopter into a nintendo DS
Select "Start wars lethal alliance.."
On the bottom screen you see either a yellow smily to the left tap or a "game" text box.
Now, you should see the list of your programs.

This is a wonderful link to get started. You don't need to read/understand everything.

Here are some homebrew tutorials. But they seem to be out of date.