I see! Thanks so much guys!
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I thought you were telling me I could put the bgm into the UTAU program to line it up but when I tried that, a message came up saying that it wouldn't go into UTAU because it wasn't an UTAU file. I guess i'm still not quite understanding...
Yeah, UTAU doesn't support BGM like VOCALOID or other vocal synths. Here's what you've gotta do;
1. Open UTAU, load your UST, select the voicebank you want and all that. Basic UTAU usage stuff, there's tons of tutorials for that.
2. Render the .wav file for the UTAU vocals.
3. Download both the original version of the song (Meaning with the vocals), as well as the instrumental version. Load them both into your DAW, in this case, FL Studio.
4. Line both the original song and instrumental up. I've never used FL Studio, but I assume it uses a drag and drop system, so just drag one of the songs around until they're perfectly lined up. You'll definitely have to zoom in to do this.
5. Mute the instrumental. Then import your UTAU vocals and line them up with the vocals of the original song. More zooming will likely be required. Then, unmute the instrumental and delete the original song. Now mix your UTAU vocals as you want.