I made my own DDR song -- which is to say, I made the steps for a song that I got elsewhere. The song is The Bad Touch, by Bloodhound Gang, and it's an excellent song which also happens to make quite a good DDR song (if I may say so myself).
Here's what you need --
- A short song. Bad Touch was 4 minutes; I cranked up Audacity and cut it to 2.5. This is still too much! It should be close to 1.5 minutes. Longer than 2 feels long.
- The BPM of your song. I just observed the waveform in Audacity and started playing the song from the beginning of a beat. I started a timer, counted the beats, and stopped right on another beat (about a minute later; the number of beats should be integral). Divide to find your BPM.
- A template .sm file. I am not sure if a blank one will work, but I suspect it will. If not, some guy on the Internet has a tutorial on how to make a StepMania song, and he has a basic template.
- A song folder. Go to your StepMania/Songs folder, find the group folder you want to put it in (or create new), and then a subfolder for your song. The name of this folder determines the sort order, so you should name it the same as the song.
Now crank up StepMania and edit your song. It's not too intuitive to learn, but it's easy to use once you've learned it.