How do I transfer GRP files from the PC to the 3DS?

Hi, it’s me again! So, I’m making some sort of a Sonic fan-game (Called Sonic SmileBit), but I’m not very good making sprites, so I was going to transfer them from the PC to the 3DS, but don’t know how to access the Ext Save Data on the PC. I know how to access the Save Data directly on the 3DS, but not in PC. Can someone help me out? I have the sprite sheet already. _{I also need help with MML...}

Petit modem can do it I think