You use it if you have a flashcart. If you have a emulator, find the directory that contains the save-files and place the file there. Then rename it to the same filename as your ROM that the save-file is from.
you mean like : A DS.nds(the original rom) then the sav file is named the same as it is or the original sav file?
aw...i could not get it right.....still. did you mean the file that contains the sav files or something else?i did tried at the battery file the GDS file that i downloaded was renamed to the same name as the sav file(1695 - Final Fantasy Revenant Wings XII(US)(Micronauts).sav)
If there was a save file before, then you have to delete it. Windows does not allow two files with the same name to be in the same folder. Then take the save file you downloaded, and convert it to the correct type using the Shunyweb converter http://shunyweb.info/convert.php Once converted, name the save file to the exact name of the old save file and put it back in the place where the old save file used to be.