1. Start using loop-based software. There are several similar programs out there (ACID, Sound Forge, FruityLoops, GarageBand, MixCraft, among others). We use loop-based software to build songs in a visual manner, chunk by chunk. A 'loop' is a short (usually 4, 8, or 16 beat) clip of sound.
2. Create several loops that sound good together. Using the aforementioned software and a USB-enabled musical keyboard, tap out a few notes on your synthesizer. If you're just starting out, it can help to borrow from one of the many loops that generally accompany such software. It's also very easy to write your own loops using just your computer's QWERTY keyboard, although we prefer to use our giant Casio CT 6000 or our tiny Akai Pro LPK25 to write the synthesizer loops in our tunes. Most loop-based software organizes the loop library by beats, key, style, etc to make it simple to sort through and find other loops that melodically fit the first loop in your creation. Also try to find a few beats (building beats by overlaying several drum patterns can be addictive! Beware!) Once you've managed to gather a few loops that sound nice together, it's time for step 3.
3. Hum a lot. Listen to your short loop-based creation made in the previous step. Listen to it a LOT. Over and over and over and over again. Hum along with it. Hum creatively, adding some rests and some harmony. Make a new mental loop using your hum. When you hear your instrumental loop, you should automatically start humming your voice part. Now replace the hum with a few words. Add some rhymes (use a free online rhyming dictionary to juice your creativity). You just made lyrics! Congratulations!
4. Record your vocals. Once you've managed to think of a voice part and written some lyrics, it's time to record. Go to a quiet area, free of white noise and reflective surfaces. We use a Snowball microphone from Blue for our vocal recording. The voice recording phase for any song can be maddening. Record more tracks than you need. Experiment with different qualities of your voice. Try singing extra breathy, or extra loud. Save all your tracks for later. Be patient, it will be worth it when you get to the next step. Remember, after you've added some effects to your vocal track it ALWAYS sounds better than you think at first.
5. Post-editing. This is the biggest step. Think of your song as a Lego creation. You have your pieces out in front of you (loops, vocals, drums) and you have to fit them together. There are hundreds of effects and sounds that can be applied to any loop. Perhaps you started out with loop that used a classical piano sound, but you change it to an old 1960's sci-fi wave sound. Then you add a spooky bongo drum part that you effected with a metallic echo. You drop in your vocals which you change to sound like you're singing from space over an old radio. When we make songs, the vast majority of time is spent in this phase. We move parts of songs around, add and subtract loops, change individual sounds, etc.
6. Beta testing. Your final step! Assuming you want to make an mp3 of your creation, it's vital that you beta test your new song on a few different systems. Some of our songs have included over 30 different tracks of audio. When all of those tracks are compressed into an mp3, the song will sound quite different. Usually the bass is much louder in the mp3 version. Send your song to a few friends and see what they think about the various volume levels. Just because it sounds good (or bad) on your computer's speakers doesn't mean it will sound okay in your car. After your beta test, go back and tweak your song a bit more until it sounds acceptable.
...and done. Enjoy your new song! We've made songs that have taken about an hour to get through all of these steps. Other songs have taken weeks. Most songs never get completed. Of the 200+ songs we've started, only about 20 have been 'done'. Don't be discouraged, and most importantly always have fun!