Erin,
Just write down a description of the video. Include the following:
Approximate Length
What you’ll provide them
What will be included in the edit (Voice narration, music cuts, animations, graphic titles)
Number of revisions that you’ll do for free.
Format of the final version
The total cost.
Then make sure you specify that the final video will be delivered once you receive full payment of all balances due.
Make sure you attach an agreement to the proposal and have them sign it and initial each page of your proposal.