Begin by logging into your Smartsheet account. Navigate to the sheet containing the data you wish to transfer. Use the “Export”� function, typically found under the File menu or in the toolbar, to save the data as a CSV file. Ensure the exported file is complete and accurately represents the data you need to move.

Install the necessary tools and libraries to interact with Google Firestore. Ensure you have Node.js installed on your computer, as it will be used to run scripts. Set up a new project directory and initialize it with `npm init` to create a `package.json` file. Install the Firebase Admin SDK by running `npm install firebase-admin`.

Log in to the Firebase Console and create a new project if you don’t have one already. Navigate to the Firestore Database section and enable Firestore in Native mode. Download the service account key by going to Project Settings > Service accounts, then click “Generate new private key.”� Save this JSON file in your project directory.

In your project directory, create a new JavaScript file (e.g., `uploadData.js`). Require the Firebase Admin SDK and initialize the app using the service account JSON file. This setup will authenticate your access to Firestore:

```javascript
const admin = require('firebase-admin');
const serviceAccount = require('./path/to/your/serviceAccountKey.json');

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

const db = admin.firestore();
```

Use Node.js to read the CSV file exported from Smartsheet. You can use the `csv-parser` package to parse the CSV data. Install it using `npm install csv-parser` and then read the file:

```javascript
const fs = require('fs');
const csv = require('csv-parser');

fs.createReadStream('path/to/your/exported-file.csv')
.pipe(csv())
.on('data', (row) => {
// This function will be called for each row in the CSV file
})
.on('end', () => {
console.log('CSV file successfully processed');
});
```

Inside the `.on('data')` callback, map each row of your CSV to a Firestore document. Determine the appropriate structure for your Firestore documents and collections. Use Firestore’s API to add documents to a collection:

```javascript
.on('data', (row) => {
db.collection('your-collection-name').add({
// Map your CSV fields to document fields
field1: row['CSV Column 1'],
field2: row['CSV Column 2'],
// Add additional fields as needed
}).then(() => {
console.log('Document successfully written!');
}).catch((error) => {
console.error('Error writing document: ', error);
});
})
```

After uploading the data, check the Firestore database in the Firebase Console to ensure all data has been correctly imported. Verify that all fields match your expectations and that there are no discrepancies. It may be helpful to write a script to fetch and log data from Firestore for a programmatic verification.

By following these steps, you can manually transfer data from Smartsheet to Google Firestore without relying on third-party connectors or integrations.