Assuming that you have already set the correct save type...
1. Open up the no$gba.ini file in your no$gba folder, and look for the line "SAV/SNA File Format".
2. Change the word "Compressed/Uncompressed" to "Raw"
3. Run the emu and save your game
4. Go to the Battery folder located in your no$gba folder, and look for your save file
5. Convert your save file using this site