Begin by exporting your data from ConvertKit. Log into your ConvertKit account, navigate to the Subscribers page, and use the Export function to download your subscribers’ data in a CSV format. Ensure that you have all the necessary fields needed for your Firestore database.

With your CSV file ready, open it in a spreadsheet editor like Google Sheets or Microsoft Excel. Review the data to ensure accuracy and consistency. Make any necessary adjustments to match the structure and schema you plan to use in Firestore.

If you haven't already, set up a Google Cloud Platform project and enable Firestore. Go to the Firebase console, create a new project, and navigate to the Firestore database section. Choose the appropriate database mode (production or test) and create a new Firestore database.

To interact with Firestore programmatically, install the Firebase CLI. Open your terminal and run the following command to install it via npm:
```bash
npm install -g firebase-tools
```
After installation, authenticate the CLI with your Google account by running:
```bash
firebase login
```

Since Firestore requires JSON format for data imports, convert your CSV data into JSON. You can write a simple script in Python or Node.js to accomplish this. The script should read the CSV file and output a JSON file structured according to your Firestore schema.

Create a script to upload your JSON data to Firestore. This can be done using the Firebase Admin SDK. Here is an example in Node.js:
```javascript
const admin = require('firebase-admin');
const serviceAccount = require('path-to-your-serviceAccountKey.json');

admin.initializeApp({
credential: admin.credential.cert(serviceAccount)
});

const firestore = admin.firestore();
const data = require('path-to-your-json-file.json');

data.forEach(async (item) => {
await firestore.collection('your-collection-name').add(item);
});
```
Replace `path-to-your-serviceAccountKey.json` and `path-to-your-json-file.json` with the actual paths to your files.

After running your import script, verify that the data has been correctly imported into Firestore. Use the Firebase console to check your Firestore database. Ensure that all fields are correctly populated and that the data matches what was in your CSV file.

By following these steps, you can effectively move data from ConvertKit to Google Firestore without relying on third-party connectors or integrations.