First, navigate to the Slack API website and create a new Slack app. Give your app a name and select the workspace you want to access. This app will allow you to interact with the Slack API and extract the needed data.

Once the app is created, you'll need to generate OAuth tokens to authenticate API requests. Go to the "OAuth & Permissions" section of your app's settings. Under "Scopes," add the necessary permissions such as `channels:history`, `channels:read`, or other relevant scopes based on the data you want to extract. Install the app to your workspace to get the OAuth Access Token.

On your local machine, set up a development environment to make API requests. You can use any programming language you're comfortable with, such as Python or JavaScript. Install any necessary libraries, such as `requests` for Python, which will help you make HTTP requests easily.

Use the Slack Web API to fetch the data you need. For instance, to get messages from a channel, make a GET request to the `conversations.history` endpoint. Use the OAuth token for authentication. Here's a Python example:
```python
import requests
url = "https://slack.com/api/conversations.history"
headers = {"Authorization": "Bearer xoxb-your-slack-token"}
params = {"channel": "your-channel-id"}
response = requests.get(url, headers=headers, params=params)
data = response.json()
```

After you receive a response from the Slack API, parse the JSON data to extract the information you need. Check for errors in the response and ensure the data is in the expected format. For instance, you may want to filter out only the messages or metadata you're interested in.

Once you have the data parsed and filtered, convert it into a JSON format if it's not already. Use your programming language's built-in JSON library to format the data correctly. In Python, you can use the `json` module to handle this conversion:
```python
import json
with open('slack_data.json', 'w') as json_file:
json.dump(data, json_file, indent=4)
```

Finally, save the JSON data to your local filesystem. Choose an appropriate directory and file name for storing your data. Ensure that your script has the necessary permissions to write to the specified location. The example in step 6 demonstrates writing the data to a file named `slack_data.json`.
By following these steps, you can efficiently move data from Slack to a local JSON file without relying on third-party connectors or integrations.