Begin by ensuring your IBM Db2 database environment is properly set up and accessible. Verify that you have the necessary credentials and permissions to access and export data from the Db2 database. Also, ensure that the Db2 command-line tools are installed on your system to facilitate data extraction.

Use the Db2 command-line interface or a script to extract the data you wish to move. You can use SQL queries to select the necessary data and use the `EXPORT` command to save it to a format like CSV or JSON. This step involves preparing the data in a format that can be easily read and processed later.

Example command:
```bash
db2 "EXPORT TO data.csv OF DEL MODIFIED BY NOCHARDEL SELECT * FROM your_table"
```

Once extracted, verify the integrity and format of the data file. Ensure that the file is correctly formatted for processing, with proper delimiters and no missing values. Depending on the size of the data, you might need to split the file into smaller chunks to manage memory and processing time efficiently.

Install and configure the Google Cloud SDK on your local machine. This will allow you to authenticate and interact with Google Cloud services, including Pub/Sub. Use the following command to install and initialize the SDK:

```bash
curl https://sdk.cloud.google.com | bash
exec -l $SHELL
gcloud init
```

Follow the prompts to authenticate and set up your Google Cloud project.

In your Google Cloud Platform (GCP) project, create a Pub/Sub topic where the data will be published. Use the `gcloud` command-line tool to create the topic:

```bash
gcloud pubsub topics create your-topic-name
```

Create a Python script using the Google Cloud Client Libraries to read the data file and publish messages to the Pub/Sub topic. Each row of data can be published as a separate message. Here's a basic example using Python:

```python
from google.cloud import pubsub_v1
import csv

# Initialize the Pub/Sub client
publisher = pubsub_v1.PublisherClient()
topic_path = publisher.topic_path('your-project-id', 'your-topic-name')

# Read data from the CSV file and publish to Pub/Sub
with open('data.csv', 'r') as file:
reader = csv.reader(file)
for row in reader:
# Convert list to a string message
message = ','.join(row)
publisher.publish(topic_path, message.encode('utf-8'))
```

After running the script, monitor the Pub/Sub topic to ensure the data is being published correctly. You can use the Google Cloud Console to view message metrics and confirm successful message publishing. Additionally, consider setting up a subscriber or using a temporary subscription to verify that messages are being received as expected.

By following these steps, you can effectively move data from IBM Db2 to Google Pub/Sub without relying on third-party connectors or integrations.