1. Create a Xero Account: If you don't already have one, sign up for a Xero account.

2. Register a New App: Go to [Xero Developer](https://developer.xero.com/), sign in, and navigate to "My Apps". Create a new app to obtain the API credentials you'll need for authentication.

3. Set the OAuth Callback URL: If you are creating a private application, you will not need a callback URL. For public and partner applications, set the OAuth callback URL to a URL you can handle responses on.

4. Generate API Credentials: Once your app is created, note down your Consumer Key and Consumer Secret (for OAuth 1.0a) or your Client ID and Client Secret (for OAuth 2.0).

1. Create a Google Cloud Project: Go to the [Google Cloud Console](https://console.cloud.google.com/), create a new project, and enable the Google Sheets API for that project.

2. Create Credentials: In the APIs & Services > Credentials area, create credentials for a service account. Download the JSON file which contains your service account's private key and store it securely.

3. Share Your Sheet: Share the Google Sheet where you want to import data with the email address of your service account.

You can choose to write your script in a language you're comfortable with. For this guide, we'll use Python as an example.

1. Set Up Your Development Environment: Install the necessary libraries for both Xero and Google Sheets APIs. For Python, you would install `requests` or `xero-python` for Xero API and `gspread` and `oauth2client` for Google Sheets API.   

   ```bash

   pip install requests xero-python gspread oauth2client

   ```

2. Authenticate with Xero: Use the credentials obtained from Xero to authenticate. With OAuth 2.0, you'll need to follow the authorization flow to get an access token.

3. Authenticate with Google Sheets: Use the service account JSON file to authenticate with Google Sheets using `gspread`.

4. Fetch Data from Xero: Use the Xero API to fetch the data you need. The Xero API documentation will be helpful to find the correct endpoints and data formats.

5. Format the Data: Once you have the data from Xero, format it as needed to fit the structure of your Google Sheet.

6. Update Google Sheets: Use the `gspread` library to select the Google Sheet and the specific range you want to update, then push the formatted data to the sheet.

If you require the data to be updated periodically:

1. Local Scheduling: Use cron jobs (Linux/macOS) or Task Scheduler (Windows) to run your script at set intervals.

2. Cloud Functions: Alternatively, deploy your script to a cloud service like AWS Lambda, Google Cloud Functions, or Azure Functions, and set a trigger for the desired interval.

1. Logging: Implement logging in your script to catch and record any errors or issues that occur.

2. Testing: Regularly test the script to ensure it's working as expected, especially after any updates to the Xero or Google Sheets APIs.

Example Script Outline (Python)

```python

import xero_python

import gspread

from oauth2client.service_account import ServiceAccountCredentials

# Authenticate with Xero

xero_client = xero_python.XeroClient(...)  # Fill in with your Xero credentials

# Authenticate with Google Sheets

scope = ['https://spreadsheets.google.com/feeds', 'https://www.googleapis.com/auth/drive']

creds = ServiceAccountCredentials.from_json_keyfile_name('service-account.json', scope)

client = gspread.authorize(creds)

sheet = client.open("Your Sheet Name").sheet1

# Fetch data from Xero

invoices = xero_client.invoices.get()

# Format data for Google Sheets

data_to_insert = [[invoice['InvoiceNumber'], invoice['Total']] for invoice in invoices]

# Update Google Sheets

sheet.update("A2", data_to_insert)  # Update range accordingly

```

Remember that this is a simplified example. You'll need to handle pagination, error checking, and other API specifics in your actual script.

By following these steps and using the provided script outline as a starting point, you should be able to move data from Xero to Google Sheets without third-party connectors or integrations.