The whole application for the Data School can feel pretty long overall, after you've built your application visualization, the next step is the phone interview.
My first tip would be maybe the most important one, know your story. Make sure you have practiced your presentation with different people, they will provide you with feedback and it will help you grow more confident as well as help you remember what you want to talk about in the different sections. Interviews can be nerve racking and coming in with extra confidence and practice is the best way to ensure your success, this is applicable to any interview you will have.
Know your topic. You have selected the topic of your first visualization yourself, you've worked on it for a while and have found a story worth presenting. There will always be facts you are less confident about but if you can know more about the topic than just what is written on your visualization. This will help you massively to once again be more confident during your presentation but also while answering questions about your topic.
Have a look at what the data school and the information lab are as well as what a consultant does. If you want to work for a company you are expected to know about what they do. This will also help you answer questions and make sure that this is actually what you want to do!
These are tips that I found helpful during my application which hopefully applies to your application too.