On the cartridges you purchase for your Nintendo DS or DSi, there's a special program file called an NDS file. This is the file that stores all of the information about your game. It's possible to download "homebrewed" (user-created) NDS files that add additional features to your Nintendo handheld device and run them as you would a game. You can easily do this using BitTorrent, a free, open-source file-sharing application.
Go to your BitTorrent site of choice (e.g., http://btjunkie.org) and search for the NDS file you'd like to download. If you're not sure what you're looking for, just type "NDS" into the search engine and see what comes up. Click on the search result you're interested in and click the "Torrent Download" link. Save the torrent file to your desktop.
Download a torrent client. For this example we'll use uTorrent from www.utorrent.com. Click the "Download" link and open the downloaded file. Follow the on-screen instructions to install the program.
Run uTorrent. Drag and drop the NDS torrent file you downloaded earlier into uTorrent and click "OK" to begin the download.
Get a flash cart. These can run anywhere from £9 to £26 and are available at many online stores, such as DSCardsDepot.com. The Acekard 2i has the best compatibility of currently available flash carts (the Acekard 2i costs roughly £13).
Connect the MicroSD card reader that came with your flash cart (it hooks up to your USB port). Insert your MicroSD card; you'll notice a new drive appear in "My Computer."
Wait for uTorrent to notify you that the NDS file has downloaded successfully; copy it to the MicroSD card by dragging it from your "Download" folder to the new drive you saw in "My Computer."
Insert the MicroSD card into the flash cart; put the flash cart into your Nintendo DS or DSi game system. Power on the system. The NDS file you downloaded will appear in a menu on the main screen. Click on it to load it and start using it.