Trim the beginning of the vocals to the point where the 1st verse or chorus(whichever is 1st)starts. Line it up to drop on time with your beat, then timestretch from the end of the audio clip until it all lines up. After you're done, slide back the beginning of the song(that you originally trimmed off) and it will be lined up too. Hope that makes since, kinda hard to explain thru type. Literally takes a few minutes total unless the original song was never created to any tempo whatsoever(live recording with no metronome)which rarely happens with songs that offer accapella versions.