Start the video and audio recordings as close to simultaneously as possible before scene shooting begins. Have one of your crew members stand with a clapper in front of the camera lens before each scene, clearly state the scene and take numbers, and clap the clapper once. Direct them to walk out of the shot and proceed with the scene.
Import the audio and video recordings to your video editing program. Drag the audio and video onto the editing program's "Timeline," which will be located at the bottom of the program window. They will appear as extended rectangles running parallel to each other. Make sure the beginning of both rectangles---the left sides---line up.
Press "Play" on the "Timeline" and listen for the clap while watching the clapper come down. Adjust the audio or video rectangle until the clap sound occurs at the exact moment the the clapper's arm connects to its body. The adjustments will cause either the audio or video clip to stick out to the left more than the other. Use the program's "Razor" or "Cut" tool---it should look like a razor blade---and click both rectangles at the point immediately following the clapper to divide them at that point. Delete the portion of the clips that contains the clapper sequence.