Mixing with acapellas

I would tell you that this is a case when live beatmatching skills are essential.

The best way I know is to listen to the song the accapella came from, so you can learn its timing and flow. Other half of the battle is to find the ideal tunes to lay said accapella on.

I think for live mashups, many will fix up their tracks in a DAW or something so they sync easily.

The key is 2 things:
I shorten my acapella to a position where I know the beat would start at a 1 count.
For example take Vanilla Ice: Ice ice baby
It is roughly 120bpm and I put my first hotcue on the “Ice” of “ice ice baby”.
So I know when my underlying 120bpm hits the 1 count I can hit the hotcue and play from there.

The next hotcues I typically put on phrases where I again now the 1, 2, 3 or 4 count of the beat (I color code those in Serato or put them into Serato Flip save zones). So basically if my underlying beat goes 1, 2, 3, 4 and I would hit the cues in that order 1, 2, 3, 4 then the song would play “normal” or as it would in the original.

That is my basic setup for a live mashup of course all tracks need to be perfectly beatmatched, not like when you do beatmatch transitions would not matter if it is .1 or even .5 offbeat, this has to be exact on spot.

If that explanation did not make any sense to you, you may have to look into basic music theory, beatcounting and bar counting first.