To begin, ensure you have access to the ActiveCampaign API. Log in to your ActiveCampaign account, navigate to 'Settings', and then 'Developer'. Here, you'll find the API URL and your API key. These credentials will be used to authenticate your requests to the ActiveCampaign server.

You will need a tool to make HTTP requests and handle JSON data. Install Python if you haven't already, and use the `requests` library to interact with the API. You can install it via pip with the command: `pip install requests`.

Create a Python script to authenticate API requests. Use the following code as a template:

```python
import requests

API_URL = 'https://youraccount.api-us1.com'
API_KEY = 'your_api_key_here'

headers = {
'Api-Token': API_KEY
}
```

Replace `'youraccount'` and `'your_api_key_here'` with your actual account URL and API key from ActiveCampaign.

Determine the type of data you want to export (e.g., contacts, campaigns). Use the appropriate endpoint to fetch this data. For contacts, the endpoint might look like this:

```python
response = requests.get(f'{API_URL}/api/3/contacts', headers=headers)

if response.status_code == 200:
data = response.json()
else:
print('Failed to fetch data:', response.status_code, response.text)
```

This code sends a GET request to the API to retrieve contact data and checks if the request was successful.

Once you have fetched the data, process it as needed. For example, you might want to extract specific fields or format the data before saving it. This step involves manipulating the JSON data structure obtained in the previous step.

```python
contacts = data.get('contacts', [])
processed_contacts = [{'id': contact['id'], 'email': contact['email']} for contact in contacts]
```

Convert the processed data into a JSON format and save it to a local file. Use Python's built-in `json` module for this purpose:

```python
import json

with open('contacts.json', 'w') as json_file:
json.dump(processed_contacts, json_file, indent=4)
```

This code saves the processed data into a file named `contacts.json` with pretty printing for readability.

After saving the data, verify the contents of the JSON file to ensure accuracy. Open `contacts.json` using a text editor or a JSON viewer to confirm that the data matches what you expected. You may also test the script by fetching different types of data or adjusting the fields you want to include in your JSON file.

By following these steps, you can efficiently move data from ActiveCampaign to a local JSON file without relying on third-party services. Make sure to handle any errors or exceptions that might occur during the API requests or data processing to ensure a smooth and reliable data transfer process.