To begin, ensure you have access to Marketo's REST API. This involves obtaining your Client ID and Client Secret from the Marketo Admin section. These credentials will allow you to authenticate and interact with Marketo's API endpoints.

Use your Client ID and Client Secret to generate an OAuth 2.0 access token. You can do this by making an HTTP POST request to the Marketo Identity URL. Store the received access token, as you will need it to authorize further API requests.

Determine the specific data you need to export from Marketo. This could be leads, campaigns, or other objects. Familiarize yourself with the necessary API endpoints and any required parameters or filters.

Utilize the Marketo REST API to fetch the data. Construct the appropriate API request with the necessary headers, including the access token for authorization. Send the request using a tool like cURL or a script in a language such as Python or JavaScript.

Once you receive the data from the API, it will typically be in JSON format. Parse this JSON response to ensure it contains the data you need. Use a programming language like Python (with libraries such as `json` or `requests`) to handle this parsing efficiently.

Depending on your requirements, you may need to transform or clean the data before saving it. This could involve filtering out unnecessary fields, renaming keys, or converting data types. Write a script in your chosen programming language to automate this transformation process.

Finally, take the processed data and write it to a local JSON file. Use file handling techniques in your programming language to create and write to the file. Ensure the file is saved with a `.json` extension and validate that the JSON structure is correct by opening and inspecting the file.

By following these steps, you can manually extract data from Marketo and store it in a local JSON file without relying on third-party connectors or integrations.