Pull the video clip onto the timeline in your editing software using your computer mouse. Adobe Premiere is popular program for Windows users, and Final Cut is a preferred Mac software. Both programs are available to consumers, and contain similar-looking timelines and editing tools. iMovie does not provide a timeline with syncing capabilities.
Find a point near the beginning of the clip where a definitive image/sound correlation takes place. Ideally this will be in the form of a clapper board called a slate. Slates have the project and shot information written on them, and are clapped together before each shot to mark the film and sound tape. This creates a clear sync mark for the editor, and is the preferred method of professional crews. If your footage does not include a slate, look for someone clapping, speaking, tapping on a desk, or any place where a sound may be precisely matched to action.
Pull the audio clip into your timeline using your computer mouse. Click on the timeline, and press the space bar to play the clip. Listen to the audio for the sound that matches the visual action you chose. Click and hold the mouse on the audio clip to grab it. Slide the audio clip around on your timeline to line up the approximate corresponding areas, trimming the clip at the beginning if needed.
Zoom in on your timeline using the zoom tool (it looks like a magnifying glass) in Adobe Premiere or Apple "+" in Final Cut. Play your clip, and slide the audio clip so that it matches the video as closely as possible.
Zoom in further on your timeline until you can see individual frames on your video clip. Scroll through the video frame by frame using your mouse to find the beginning of the action the audio should match. For example, if you are matching the audio and video at a slate, find the frame where the slate claps together.
Expand the audio clip by dragging the top edge of it up so you can see the waveform. Find the beginning of the audio, or the "clack" sound if you are matching to a slate. Slide the audio clip to match the video frame you chose. Play your audio and video together to be sure it matches exactly, and adjust the audio if needed.
Link the video and audio clip together by selecting both clips (hold the shift key down while you select each clip) and select "link" from the Clip menu in Adobe Premiere, or press Command L in Final Cut. The tracks will now behave as one clip while you edit your scene.