Purchase or download a legal copy of a music mixing program such as Virtual DJ or Zulu and install it onto your computer.
Load your music library into the program, using a simple drag-and-drop method.
Connect your computer's sound card into Channel 1 Input on the mixer. Connect your CD player into Channel 2 Input. Connect the mixer output into the main speaker input.
Use the tap tempo button if your DJ program has one. According to Numark Industries, automatic BPM, or beats-per-minute, calculators can potentially misinterpret the tempo of a song, so you should manually catalog the tempo for smaller collections.
Load one song into Side A of your DJ program. Load another song with similar BPM -- +/-3 -- into Side B.
Press the Play button in Side B, when the first song approaches its end. Slowly move the crossfader bar in the program to slowly shift from Side A to Side B. The crossfader should look like a horizontal slider bar. A crossfader gradually lowers the volume on one channel and raises it on the other. As the song in Side B plays, load another song into Side A.
Use the channel pitch shifting bars on the mixer or within your program to gradually speed up or slow down the track to blend smoothly into the next song, if you have no songs that match the BPM of the current song.
Start the desired track on the CD and slowly shift the crossfader on the mixer from Channel 1 into Channel 2, to switch to your CD player.