Prepare to edit!
Before you can edit a video, you must first collect all the files you're going to use, by adding them to the "Collection" area. Begin by importing a video file. This can be done in two ways:
The video file has its own audio embedded, but if you want to overlay a song to play along with the video, instead of using the attached audio file, then you need to import the music file you want too. Click on "Import audio or music" from the task lists on the top left of the screen, browse to the file, click on it and then click on "Import".
As you add the files, they will appear in the area labelled "Collection". The video files will be identified by a movie reel border and the music files by a music note in the icon. You can add as many files to the collection as you want. If you decide not to use a file anymore, just click on it and delete it (this will not delete the file from your computer; merely from the collection area).
Now to start compiling your video: first, drag and drop the video file that you want to begin with to the linear box below that has a timeline above it. After you dragged and dropped the video file, you'll see that it now suddenly shows a "video" linear bar, with an "audio" linear bar below it.
Windows Movie Maker basically splits the file you added into two components: the visual part and the audio part. The two parts are connected though — should you change the video, the audio will change with it. However, you can silence the audio that accompanies the video by right-clicking on the horizontal audio box in the timeline and then selecting "mute".
If you want some music to accompany your video, drag and drop the music file to the space next to "Audio/Music" in the timeline.
Note that it will not automatically start at the beginning of the timeline, but will sit where you let go of the mouse-button. If you want the music to begin playing from the start, then click on the music bar that you've just added to the timeline, and drag and drop it to the left to the beginning of the timeline.