Over the past three days, I've learnt so much about producing mashups. I think that one of the best ways to learn is to actually do, and these recent experiences have proven vastly useful indeed.
1. Try not to tamper with good drops inherent in well-known songs; everyone expects that good drop.
2. Amplify sounds that need amplifying.
3. Splice tracks and drop the beats. It is not necessary to force entire rounds of 4 into the musical grid if it doesn't make any sense.
4. Manually assess the BPM of acapellas by timing a round of 4x4 or 8x4 (or whatever you want), comparing that to a similar set of beats in the EDM track, and do the mathematics accordingly. Do not rely on BPM provided by programs. After this, change the tempo of the acapella (without changing the pitch) to suit the EDM track's beat exactly. For greater accuracy, find out the EDM track's exact BPM manually as well. If comparing the relative difference between the acapella's BPM and the EDM track's BPM, then the exact BPM's of both tracks need not be known. The acapella's BPM can be changed to suit the EDM track's (by changing the acapella's BPM percentage-wise using Audacity or some other music program). The advantage of this latter method is that the EDM track need not undergo a tempo-change at all.
The simple mathematics: For a similar set of beats between acapella and EDM,
Divide the acapella timing by the EDM track's timing, then multiply that by 100%.
Proceed to modify the acapella's tempo value by this exact percentage.
EDM track does 8x4 in 15 seconds
Acapella does 8x4 in 30 seconds
30/15 x 100% = 200%
Increase the acapella by 200% (or a factor of 2) and you'd have perfect beat-matching between the two tracks.
The mashup can now begin!