First, you need to gain access to your Amazon Ads data. This can be done by using the Amazon Advertising API. You must create a developer account and register your application to get access credentials. With the appropriate credentials, you can make authenticated requests to the API to pull the necessary data. Familiarize yourself with the API documentation to understand the endpoints and data structures.

Write a script (using a language like Python, Java, or Node.js) to extract data from Amazon Ads. Use the API's endpoints to fetch the required datasets. Ensure that your script handles authentication using the credentials obtained in the first step, and appropriately manages pagination and rate limits to efficiently handle large datasets.

Once data is extracted, the next step is to transform it into a JSON format. Firestore uses JSON-like documents, so converting your data into this format will make the subsequent steps more straightforward. Ensure that each record is structured correctly, with field names and types that match your Firestore database schema.

Before uploading data to Firestore, set up your Google Cloud environment. This involves creating a Google Cloud project if you haven't already, enabling the Firestore API, and setting up authentication. Use a service account with the necessary permissions to interact with Firestore, and download its key in JSON format for use in your script.

In your script, establish a connection to Firestore using the Google Cloud Client Library appropriate for your scripting language (e.g., `google-cloud-firestore` for Python). Load the service account credentials to authenticate and initialize the Firestore client. This will allow your script to interact with Firestore securely.

With the Firestore client set up, iterate over your JSON-formatted data and upload it to your Firestore database. Use the appropriate Firestore methods to create or update documents in your specified collections. Ensure that you handle exceptions and manage batches of writes to optimize performance and maintain data integrity.

Finally, automate the process to ensure that data from Amazon Ads is regularly updated in Firestore. You can use cron jobs or cloud-based task schedulers to run your script at defined intervals. This will help maintain up-to-date data in Firestore without manual intervention, ensuring that your data pipeline is robust and reliable.

By following these steps, you can efficiently move data from Amazon Ads to Google Firestore without relying on third-party solutions.