First, you need to create a Plaid account if you haven't done so already. Visit the Plaid website and sign up for an account. Once registered, log in to your Plaid dashboard to create a new application, which will provide you with client credentials (client ID and secret) necessary for API access.

You'll need to interact with the Plaid API using a programming language. For Python, you can install the Plaid client library using pip. Open your terminal and run:
```sh
pip install plaid-python
```
This library will help you send requests to Plaid's API and handle responses.

In the Plaid dashboard, navigate to the API section to obtain your client ID, secret, and a public key. These credentials are crucial for authenticating requests. Ensure that you're using the correct environment (sandbox, development, or production) as per your testing needs.

Use the Plaid client library to create a link token and simulate a user login to obtain an access token. This token allows you to retrieve user-specific data. Here is a basic Python script to authenticate and retrieve an access token:
```python
from plaid import Client

client = Client(client_id='YOUR_CLIENT_ID', secret='YOUR_SECRET', environment='sandbox')

response = client.Item.public_token.exchange('YOUR_PUBLIC_TOKEN')
access_token = response['access_token']
```

With the access token, you can now call the Plaid API to fetch data. For example, to get transaction data, you can use:
```python
response = client.Transactions.get(access_token, start_date='2022-01-01', end_date='2022-01-31')
transactions = response['transactions']
```
Adjust the date range as needed to get the desired data.

The data retrieved from Plaid is typically in JSON format, but you might want to process or filter it before saving. You can use Python's built-in `json` library to format this data:
```python
import json

json_data = json.dumps(transactions, indent=4)
```

Finally, save the JSON data to a local file. Create a new JSON file and write the formatted data into it:
```python
with open('transactions.json', 'w') as file:
file.write(json_data)
```
This creates a file named `transactions.json` in your current working directory, containing the data you fetched from Plaid.

By following these steps, you can efficiently move data from Plaid to a local JSON file without relying on third-party services.