Start by ensuring your development environment has the necessary tools. Install Python and the `google-cloud-pubsub` library, which allows you to interact with Google Pub/Sub. Also, set up authentication by downloading a service account key from your Google Cloud Console and setting the `GOOGLE_APPLICATION_CREDENTIALS` environment variable to the path of this key.

Identify the exchange rates API you will use and obtain any necessary API keys. Use Python's `requests` library to send an HTTP GET request to the API endpoint. For example, using `requests.get(url, headers={'Authorization': 'Bearer YOUR_API_KEY'})` to fetch data.

Once you receive a response from the API, parse the JSON content to extract the exchange rate data. Use Python's built-in `json` library to handle the JSON response. For example, `data = response.json()` followed by accessing specific fields like `data['rates']`.

Transform the data into a message format suitable for Google Pub/Sub. Convert the relevant data into a JSON string using `json.dumps()`, ensuring that the message format aligns with any schema you plan to enforce in your Pub/Sub topic.

In the Google Cloud Console, create a new Pub/Sub topic that will hold the exchange rate messages. Use the `gcloud` command-line tool or Google Cloud Console to create a topic, for example, `gcloud pubsub topics create exchange-rates`.

Use the `google-cloud-pubsub` library to publish the transformed data to the Pub/Sub topic. Create a Publisher Client in Python and use the `publish()` method to send the message. For example:
```python
from google.cloud import pubsub_v1

publisher = pubsub_v1.PublisherClient()
topic_path = publisher.topic_path('your-project-id', 'exchange-rates')
future = publisher.publish(topic_path, data=message.encode('utf-8'))
future.result()
```

To keep the exchange rates data updated, automate this process using a cron job or a scheduling library like `APScheduler` in Python. Set the script to run at regular intervals, such as every hour, to fetch the latest data and publish it to Pub/Sub.

By following these steps, you can effectively move data from an exchange rates API to Google Pub/Sub, enabling real-time data processing and integration into other Google Cloud services.