First, manually export your data from Smartsheet. You can do this by opening your Smartsheet document, navigating to the "File" menu, and selecting "Export." Choose the format that suits your needs, such as CSV or Excel. Save the file to your local system.

Ensure you have the Google Cloud SDK installed on your local machine. This tool will allow you to interact with Google Cloud services. You can download and install the SDK from the official Google Cloud website. After installation, authenticate by running `gcloud init` and follow the prompts to log in with your Google account.

In your Google Cloud Console, navigate to the Pub/Sub section. Create a new topic by clicking on "Create Topic." Give your topic a unique name. This topic will serve as the destination for your Smartsheet data.

Develop a Python script to parse the exported Smartsheet data. Use Python's built-in libraries, like `csv` or `pandas`, to read the file. Ensure your script can handle the data format (CSV, Excel) appropriately, extracting the necessary fields for publishing to Pub/Sub.

Extend your Python script to publish data to your Pub/Sub topic. Use the `google-cloud-pubsub` library for this purpose. Install the library using pip (`pip install google-cloud-pubsub`). Authenticate your script using a service account JSON key with Pub/Sub permissions. Publish each row of your parsed data as a message to the Pub/Sub topic.

Example snippet to publish a message:
```python
from google.cloud import pubsub_v1

publisher = pubsub_v1.PublisherClient()
topic_path = publisher.topic_path('your-project-id', 'your-topic-name')

def publish_message(data):
future = publisher.publish(topic_path, data.encode('utf-8'))
print(f'Published message ID: {future.result()}')

# Example call
publish_message('Your data here')
```

Implement error handling in your script to manage potential issues during data publishing. Use try-except blocks to catch exceptions, log errors, and ensure your script can continue or retry publishing messages in case of failures.

To automate the data transfer, schedule your Python script to run at regular intervals using a task scheduler like cron (Linux/Mac) or Task Scheduler (Windows). This ensures your data is consistently updated in Google Pub/Sub without manual intervention.

By following these steps, you can effectively transfer data from Smartsheet to Google Pub/Sub using a custom solution.