Syncing acapella to preexisting instrumental

[ohmygod]

Yes, I know there's a lot of information out there about syncing acapella tracks, but from all my searching I've only found out how to capture the acapella bpm FIRST and then build an instrumental around it. What I want to do is insert an acapella into a pre-made instrumental, but every time I do, the warping never works correctly. I can't get it to fit. I'm able to do it the other way just fine. What am I missing? Or can someone point me to a tutorial that can explain this?

[longjohns]

just put markers on the acapella where the beats should be

try to use as few as possible