How do you make a explainer video for your Product?

Recently we made our first explainer video for Curatemails.
It took us all day to prepare it. In the process of creating it, we faced lot of challenges:

• What content should go in the video.
• What should be the duration of the video.
• Should we include voice or the text

If you have created explainer video for your product/service in the past, how do you answer this?
Is there any rule book for it?
How do you come up with an ideal explainer video?