So some guys are finding it difficult to work with the roms so here it is a tutorial complete with a link to the file.
No 1- Download the software that you need to patch DS games to run on every GBA Flashcart, from HERE or click Here and download the file ndspatch.rar .
No 2- Extract the files into an empty folder (preferably), and copy the CLEAN rom dumps/backups/demos there.
No 3- Drag the roms you want to convert on the .exe file ndspatch.exe .
No 4- You will see that new .nds.gba files have been created. These are trimmed and patched nds games, so if you have a normal gba flashcart use the files as is in .nds.gba format. If you own a supercard or M3 or something that is able to play it's own patched roms in DS mode you need to rename the file into .nds instead of .nds.gba format.
No 5- Now copy the roms onto your flashcart, and run them. BUT you need to have either PASSME/FLASHME/WIFIME/SUPERPASS available in order to run these games, so if you only have a flashcart but none of the above you cannot run the games, and will result in white screen.
No 6- To be able to save you need to copy a .sav file from a converted gba game(in the case of the supercard users) or a normal .sav file. Now rename the save file exactly as the converted ds rom you want to associate/assign it to.
No 7- If you succeeded in loading the ds rom play it as usual, but then the saves will be redirected to the SRAM, so you have to quickly turn ON and OFF the DS, boot in DS mode, and save the SRAM to the gba.sav file. (In the case of supercard users you have to go to the saver menu and select the .sav file then select yes and it's done)
No 8- I am not able to tell if other flashcards then Supercard automatically load the save files, but it should be enabled in order to load the save file in the DS rom.( For Supercard users just check the AUTO LOAD SAVER option from the OPTIONS menu when in DS mode)
NOTE: Not all roms patched with the ndspatcher work, save/save correctly. For example, WARIO WARE = 2 white screens, NEED FOR SPEED UNDERGROUND 2 + PGA TOUR= NOT saving, and NINTENDOGS CHIUAUA (US) = SAVING BUT NOT ALL DATA (because of some limitations in the saving)