Playing ROMS on your DS Lite or DS
1. Once your flashcart kit arrives, unpack it and remove the DS cartridge together with any additional accessories.
2. Go to the manufacturer’s website in order to find the correct firmware version for your flashcart – updating to the latest firmware ensures that you aren’t left behind trying out a new game that doesn’t seem to work.
3. Updating the firmware is a simple act of copying over the correct files to the Micro-SD card, just plug in your USB adapter and copy over all the files in the firmware pack over to the root of the memory card.
4. Once all the firmware files have been updated, create a new folder in the SD card’s root called “Games".
5. Now the Games folder is ready to accept your ROMS (which should all end in *.DS)
6. Proceed to this website and find what ROM you’re looking for and download it.
7. Once you’ve downloaded the ROM, unpack it using 7-Zip or WinRAR and find the *.DS file
8. Place the *.DS file into your “Games" folder on the SD card’s root and you can start playing the ROM after booting up the flashcart.
And that’s all there is to it – I’ll leave it up to you on whether or not you should be doing this at all. While the resources are there, the legality of the issue is that it’s unquestionably piracy to do so.