Move signups from Firebase to Mailchimp using Zapier

I need help setting up a Zap in Zapier. I have an App (in test phase) - when a user registers the data is stored on Firebase. I would like the data to be automatically uploaded to an Audience on my Mailchimp account.
I am happy to share my screen with you via AnyDesk so that I can see and learn how you do this.

I already have a Zapier account and have verified my Firebase and Mailchimp accounts on it.