Click the "File" pull down menu in the recording software. Select the "Import" option to open the file selection interface. Navigate to the first desired MP3 file and select. Click the "Open" button.
Repeat Step 1, however select the second MP3. Continue to repeat until each desired MP3 file is open in the recording software. Select each file, right click them, and select the "Properties" tab. Look for the tempo for each song in the properties interface and note if any are the same tempo.
Select the "Lock" button next to each MP3 of the same tempo. Select each track that is not in that tempo. Click the "Edit" pull down menu. Scroll down to "Edit Speed" tab and select. In the tempo text box, enter the tempo number of the locked tracks, and select "OK."
Continue this edit to each non-locked track. Listen to the song to ensure each song is synced up in the new mashup file.