Launch Mixcraft. Click the icon next to "Build Loop and Beat Match Music" to start a new mashup session.
Double-click the first audio track. Navigate to the folder containing the instrumental track and double-click it to insert it into the session. Click "Yes" when Mixcraft asks you if you want to change the project's tempo.
Click the "Sound" tab near the bottom of the window. Note the letter next to "Key." Click the "Project" tab and select the same letter from the "Key" drop-down menu.
Double-click the second audio track. Double-click the acapella file to add it to the session.
Click the "Sound" tab. Click the "Adjust Pitch By" drop-down menu and select "Adjust to Project Key."
Click "Play" to listen to the mashup. If you're satisfied with the result, click "File" and hover the mouse over "Mix Down To." Click on an audio format, type in a name for the mashup and click "Save."