Download, install and open a video-editing program with advanced audio controls (see "Resources" below).
Open a new project and select the parameters for the video clip. For example, if the clip was shot in 720p, create a project using a 720p template. All video-editing programs offer project templates.
Load the video file into the program.
Drag the video clip into the "Effects" window; every program has an effects window.
Open the "Audio Effects" menu and locate the "Channel Volume" effect. Drag this effect into the clip in the "Effects Window."
Set "Key frames" in the clip. Set key frames at the start and end of the pan. Set key frames for both channels.
Go to the first key frame and decrease the volume for that channel until a desired volume is reached. The channel that is decreased is the one that should be quiet. For example, if no sound is supposed to be emitted from the left side of the screen, decrease the left channel.
Go to the last key frame and increase the volume for the channel with previously decreased volume. On the channel with unaltered volume, decrease the volume to a desired amount. The clip will now have a surround-sound pan for that scene.