Begin by logging into your HubSpot account. Navigate to the "Contacts" or "Deals" section, depending on the type of data you wish to export. Use the export feature to download the data as a CSV file. Ensure you have the correct permissions to export data and select the appropriate fields for export.

Open the exported CSV file in a spreadsheet application like Excel or Google Sheets. Review the data and clean it if necessary, removing any unnecessary columns or rows. Ensure that the data types are consistent and ready for import into Firestore. Save the cleaned file as a CSV to ensure compatibility.

Go to the Google Cloud Platform Console and create a new project if you don’t have one already. Enable the Firestore API by navigating to the "APIs & Services" section, then to "Library," and search for "Firestore API" to enable it.

Within the Google Cloud Console, navigate to Firestore and create a new database. Select "Start in production mode" or "Start in test mode" based on your requirements. Choose a location for your Firestore database, preferably the region closest to where your application will run.

Install the Google Cloud SDK on your local machine. This will allow you to use command-line tools to interact with Google Cloud services. Once installed, open a terminal or command prompt and initialize the SDK by running `gcloud init`. Follow the prompts to authenticate and set your project.

Create a Python script (or use another programming language of your choice) to read the CSV file and write the data to Firestore. Use the `google-cloud-firestore` library to interact with Firestore. Your script should read each row of the CSV, create a Firestore document, and batch write the data to the database to optimize performance.

Example Python snippet:
```python
from google.cloud import firestore
import csv

# Initialize Firestore client
db = firestore.Client()

# Open and read the CSV file
with open('path/to/your/file.csv', newline='') as csvfile:
datareader = csv.DictReader(csvfile)
batch = db.batch()

for row in datareader:
doc_ref = db.collection('your_collection_name').document()
batch.set(doc_ref, row)

batch.commit()
```

Execute the script to import your data. Monitor the process for any errors or warnings. Once the script completes, verify that the data has been successfully imported into Firestore by checking the Firestore console. Review the data structure and ensure that all fields have been imported correctly.

By following these steps, you'll be able to migrate data from HubSpot to Google Firestore without relying on third-party connectors or integrations. Remember to handle sensitive data carefully and comply with any relevant data protection regulations.