Write a 2-column script for your video.
In the left-hand column of a two-column script you should include the visual aspects of the production. Here is where you identify what people will see on the screen. This includes backgrounds, characters, and key words.
The right-hand column contains the audio portions of the production. Write the dialogue, sound effects and music in this column.
The content of the columns should be synchronized, so that the dialogue in the right column matches the shots in the left column.