Find an instrumental and acapella track. Many can be purchased individually on iTunes or other music download services and websites. The cost for one track is miniscule and you can use the track over and over for different kinds of mashups.
Load the instrumental track into ACID Music. The simplest way to accomplish this is by clicking on the "Explorer" tab in the bottom left-hand corner of the main window and scrolling to the folder where the instrumental track is located. Double click on the file. Since it's the first time that the song has been loaded into ACID, the Beatmapper Wizard pop up window will appear and ask if you want to use it on the file. Choose "Yes" and click "Next."
Calculate the BPM (beats per minute) of the song. Beatmapper will display a waveform of the music file. Find the first downbeat and set the line cursor next to it by clicking the top space above the waveform. The cursor will blink in the position it's been placed. Click "Next."
Listen to the highlighted portion that Beatmapper displays by either clicking the "Space" bar or the "Play" icon on the wizard. One bar of the song will loop. If the tempo is off, adjust it by placing the mouse at either side of the top of the waveform. There is a dark blue bar with yellow corners. Hover the cursor there until it transforms into a two-sided horizontal arrow then move it left or right as needed. When finished, click "Next."
Verify the tempo by listening to several measures of the song. Beatmapper will ascertain the BPM of the song and display it in the bottom left under the waveform. Note the tempo and then click "Next." A box will display with three checkboxes. Make sure all three are checked and click "Finish."
Repeat the steps for the acapella track. Align, arrange, and edit the track with the verse and chorus sections of the instrumental. Save often. When finished, click "File," "Render As," choose the file format (MP3, WAV), name the song and click "Save."