Write your video script in two columns, with the audio of the script (sound effects included) in one column and the images (or video) in the other. The audio and video will run side by side, exactly as they will appear together in the finished script. Movie Magic Screenwriter and Final Draft have audio/video templates to make formatting easy.
Think visually and keep things simple. A video is primarily the visual imagery on the screen. You want to write images succinctly. A director should be able to glance at the script and immediately see the image you want to convey.
Focus on your target audience while you write your script. A video typically sells a product or showcases something. If you're writing a commercial script, tailoring your image descriptions to your target audience will create a more effective, easier-to-shoot video script.
Keep any dialogue in your video script (notated in the audio column) in the background and the visuals in the foreground. Unlike a movie script, the video script can include camera angles and shots to help bring the visual aspect of the video to life.