Begin by logging into your PartnerStack account. Navigate to the section where your data is stored, such as reports or analytics. Look for an option to export your data. Typically, this will allow you to download the data as a CSV or Excel file. Ensure you export all necessary data for your needs.

After exporting the data, open the file using a spreadsheet application like Microsoft Excel or Google Sheets. Review the data to ensure it is clean and well-organized. Make any necessary adjustments, such as formatting columns, removing duplicates, or correcting errors. Save the file in CSV format for compatibility with later steps.

Create a new project in the Google Cloud Console if you haven't already. Go to the Google Cloud Console, click on the project drop-down and select "New Project." Give your project a name and note the project ID for future reference.

With your project selected, navigate to the "Firestore" section in the Google Cloud Console. Click on "Create Database" and select the Firestore mode you prefer (Native or Datastore mode). Follow the prompts to complete the setup, ensuring the database is ready to receive data.

Install the Firebase Command Line Interface (CLI) on your local machine. This is essential for deploying data to Firestore. Open your terminal and run the command:
```
npm install -g firebase-tools
```
Ensure Node.js and npm are installed on your system before running this command.

Create a small script using a programming language like Python or Node.js. This script should read data from your prepared CSV file and upload it to Firestore. Here is a basic example in Node.js:
- Install Firebase Admin SDK:
```
npm install firebase-admin
```
- Initialize Firebase Admin in your script:
```javascript
const admin = require('firebase-admin');
const csv = require('csv-parser');
const fs = require('fs');

admin.initializeApp({
credential: admin.credential.applicationDefault(),
databaseURL: 'https://.firebaseio.com'
});

const db = admin.firestore();

fs.createReadStream('data.csv')
.pipe(csv())
.on('data', (row) => {
db.collection('your-collection-name').add(row)
.then(() => console.log('Data added:', row))
.catch((error) => console.error('Error adding data:', error));
})
.on('end', () => {
console.log('CSV file successfully processed');
});
```
Replace `` and `your-collection-name` with your actual project ID and desired Firestore collection name.

Execute your script from the terminal to begin the data upload process. Monitor the terminal for any error messages and ensure each row is processed successfully. Once complete, go to the Firestore section in Google Cloud Console to verify that the data has been uploaded correctly. Check a few entries to ensure the data integrity is maintained.

By following these steps, you can efficiently move data from PartnerStack to Google Firestore manually. Adjust the script logic as necessary to fit your specific data structure and requirements.