I hate to tell you this but I have been doing remixing of songs for awhile, there is not an automatic way to be honest especially if you dont have a straight ahead beat and rhythm, these programs will get you in the ballpark but just as big rob says the flow is a lot, and it sometimes kills me because even the people that sometimes judge remixes cant tell if it is always on , especially with a rapper that either has no consistent timing or flow.....tapping out and using about two bars of the acapella looped can sometimes help you, get the beat going and loop it then put the part of the acapella in there you wanna try and match up, go a little further and take about 4 bars and do it........then after you have a pretty good estimate of the bpm.....make the beat......then play the acapella back from an external input a few times....then I would line everything up in protools or whatever you use....thats about all I can tell you.....but once you get it this close its sometimes a matter of where you align the vocals in relation to the first downbeat, you sometimes need to listen to the original and judge the emcee's inflection or emphasis in relation to the original beat's kick and snare...this can help you out for vocal placement.....