Import the video into your movie editing software.
Import the song into your movie editing software. In Windows Movie Maker, there is an Import Audio or Music option under Capture Video in the Move Tasks pane. Most programs will have something similar. If you're having trouble you can find the exact location of yours using the help menu. This will add the song to your movie-making program but it will not add it to your video.
Drag or copy the song into the video timeline. This tells the program when you want the music to start and stop. You can move the song around once it is in place and you can also shorten the space it plays in. If you need to cover more space, then reinsert the song from the beginning again, rather than stretching the end as sometimes stretching the song will insert silence after the song finishes.
Open the mixer for your song and make sure the volume is set for the correct level. In Windows Movie Maker, you do this by clicking on Volume in the timeline window. Move the sliders or adjust the knobs until your music is at the right level. You can preview the video with the music by pressing play.
Finish editing the other aspects of your video and then export it using the export command in the file menu.