First, create an account on the IP2WHOIS website if you haven't already. Familiarize yourself with the API documentation provided by IP2WHOIS to understand how to make requests and retrieve data about domain information.

Log in to your IP2WHOIS account and navigate to the API section. Obtain your API key, which will be required for authenticating requests made to the IP2WHOIS service.

Go to the Google Cloud Console and create a new project or select an existing one. Ensure that you have billing enabled for your project to use Google Firestore.

Once your project is ready, navigate to the Firestore section in the Google Cloud Console. Enable Firestore in Native mode to use it for storing and managing your data.

Set up your development environment by installing the necessary libraries. For a Python script, you would typically use `requests` for HTTP requests and `google-cloud-firestore` for interacting with Firestore. Install them using pip:
```
pip install requests google-cloud-firestore
```

Create a script that will:
- Use the `requests` library to make HTTP GET requests to the IP2WHOIS API, using the API key for authentication.
- Parse the JSON response to extract the desired data fields.
- Use the `google-cloud-firestore` library to connect to your Firestore database and insert the parsed data as documents in a collection.

Here is a basic structure in Python:
```python
import requests
from google.cloud import firestore

# Initialize Firestore
db = firestore.Client()

# Function to fetch data from IP2WHOIS
def fetch_ip2whois_data(domain):
api_key = 'YOUR_IP2WHOIS_API_KEY'
url = f'https://api.ip2whois.com/v1?key={api_key}&domain={domain}'
response = requests.get(url)
return response.json()

# Function to push data to Firestore
def push_to_firestore(collection, data):
doc_ref = db.collection(collection).document(data['domain'])
doc_ref.set(data)

# Example usage
domain_info = fetch_ip2whois_data('example.com')
push_to_firestore('domains', domain_info)
```

Run your script to ensure that data is being retrieved from IP2WHOIS and stored in Firestore correctly. Verify the data in the Firestore console. Implement error handling and logging in your script to manage any issues that arise during execution. Regularly monitor the data transfer process to ensure ongoing accuracy and performance.

This guide provides a straightforward approach to transferring data from IP2WHOIS to Google Firestore using a custom script without relying on third-party connectors or integrations.