By no means is this the correct way to do a mashup. It is just how I do it which is why they are called "Badly Made Mashups". I'll try and explain how it should be done as I go along.
Firstly I grab a mp3 of the acapella and instrumental I intend to use. These are usually low quality (128kbps) found through searching Google, Yahoo, and Bing. The correct way would be to start with higher quality sources - possibly using studio stems and files in a higher quality format (such as wav, and aiff to name a couple).
Then I find out what the BPM (Beats Per Minute) of each track is, which is usually just doing a search on Google with artist, title, and bpm (for example: Michael Jackson Smooth Criminal bpm). There is software to calculate the BPM correctly and I'm almost positive that an overall BPM does not take into consideration various sections of a track.
Then it is matching the acapella to the instrumental with regards to the BPM. I'm sure the method I use is incorrect but for my ears it works well enough. I'm sure the professional mashup artists would shudder at the method. For example:
Acapella: Icona Pop - I Love It (BPM 126)
Instrumental: Michael Jackson - Smooth Criminal (BPM 118)
In this case the acapella would need to be 'slowed down' to match the BPM of the instrumental. And the math for that is:
126 /118 = 1.0677966101694915254237288135593. Which I round up and convert to a percentage - 1.06779 or 106.78%
Then using Audacity I stretch the acapella. With the above acapella, which is 2 minutes 30.335 seconds long, it needs to be slowed down to match the instrumental. Audacity handles speed as percentage. In this case, as it needs to be slowed down it would be -6.78% change. If it was to be sped up (when the acapella is slower than the instrumental) it would be a positive number. For the acapella in this example it stretched it to 2 minutes 41.269 seconds.
Then save a copy of the file just so the original file stays at its normal speed for possible future use.
Then create a new file, and add 4 stereo tracks. Paste the instrumental into track 1 and the acapella into track 2. Track 3 and 4 are used for pasting sections of the acapella into.
After you have cut and pasted the various acapella pieces into new tracks and are happy with how everything sounds delete the unused acapella track (Track 2 in this example) and export the whole thing. I usually export as a 320kbps mp3 but the choice is yours.
That is it. As I said it is in no way how a mashup should be created. It is just the way I do it. Also there are a load of other things to do usually like adding echo to certain parts. This is just a rough guide to how I use Audacity. The best thing to do is use Audacity and play around with it.
Audacity is available for Windows, Mac, and Linux and can be downloaded at: http://audacity.sourceforge.net/
