If you have a whole song in mp3 or any other format it is extremely difficult to split into multiple tracks or take out instrument sounds to make a key-sound bms, without completely ruining the quality.
As for non key-sound, it's not that hard, actually it's very easy.
Bmse is the best bms editor out there, try looking at bms of different songs, and try to learn from them as much as you can.
Once you get the tempo right all notes should fall into place easily. |