Connect your MIDI cable to the output of your synthesizer and the input of your sound card. You may need to purchase a separate MIDI adapter to connect the MIDI cable and sound card. Alternatively, you can purchase a "MIDI to USB" cable which will hook directly into the USB port on your computer.
Initialize your audio editing program. Open the "Audio" menu, and select the synthesizer as your input device. In most audio editing programs, this option will be listed as "Use MIDI Instrument for Input."
Press "Record" on your audio editing software. Record a short musical sample by playing it on the synthesizer. The audio will be translated into a digital format -- capable of playback through your audio editor. Record five audio excerpts to start. Each excerpt only needs to be a few seconds long.
Turn one of the audio excerpts into a rhythm track by selecting the "MIDI Instrument" option from the "Audio Console" at the left of the audio track. Choose from snare drums, cymbals, bass drums or any number of available rhythm instruments.
Change the other four audio excerpts into melodic instruments. You can choose from 128 instruments, if you are using general MIDI. General MIDI is a standardized set of instruments, where each instrument is given a specific number. For instance, in general MIDI, selecting the number 1 will give you an "Acoustic Grand Piano" on any computer system.
Copy and paste your rhythm track by highlighting the entire audio sequence in the "Track View" and pressing "CTRL+C." Then paste the sequence 10 times by pressing "CTRL+V" 10 times. This will give you a rhythm track for the entire sequence.
Listen to your rhythm track and modify it as you see fit using the options under the "Audio Effects" menu. You can choose from several options. "Retrograde" will play your rhythm backwards. "Increase Tempo" will speed up the rhythm. "Change Pitch" will change the pitch higher or lower.
Paste the other four audio samples in ways that makes sense logically at various points throughout the audio sequence. Once you have pasted a synthesized sound into the preview window, you can drag each excerpt -- pulling on the left or right of the excerpt to make it longer or shorter -- as well as organize and duplicate your ideas to create an audio mix of your sounds.
Save as an audio file by selecting "Export to Audio" or "Bounce to Audio" depending on your program. Choose a WAV audio format for the best quality sound and to record a CD. Choose MP3 if you wish to distribute the file online.