Instructions
Download a Nintendo DS emulator. A few examples of such applications are DeSmuME, iDeaS and Dualis (see Resources). Be sure to save the file in a place you can easily find it.
Start the emulator program by double-clicking the file you downloaded.
Select "File" from the main menu using your mouse and click on the "Load Game" or "Load ROM" option.
Navigate to the location where you've saved your homebrew Nintendo DS games. Click on the game's file and press the "Open" button.
Control the game using the arrow keys (in place of the d-pad), the Z key (in place of the B button) and the X key (in place of the A button). Press "A" for the left trigger and "S" for the right trigger. For most Nintendo DS emulators, "Enter" serves as the "Start" button and the space bar acts as the "Select" button.