Pick the music. In most cases, the music you select should not be too distracting to the viewer. Make sure the music file is on your hard drive. If you load the file directly from a CD, it will disappear from the project once you eject the disc.
Convert the music file. Most editing software can only work with a few types of files. For instance Final Cut Pro requires AIFF files, while iMovie can use WAV or MP3. Check the software manual. Converting to the right type is fairly simple. Most music programs like iTunes or Musicmatch Jukebox have a convert option. Go to the options menu, choose the file type for the conversion, select the file and click convert.
Import the song into your project. Most editing programs have some kind of bin for video and audio clips. Just drag the file into this bin. It may take a minute or so to load. Now, find the point where you want the music to start in the timeline, which is where you arrange the order of video clips. Drag the file down to that point. Now, the music will play during the video.
Perfect the arrangement. You can work with the music just like a video clip. Cut pieces out. Adjust the volume. Fade in or fade out. Have it peak at certain points. When you get it just right, save your project. You now have a soundtrack.