Begin by accessing the Google PageSpeed Insights API. Register for an API key via the Google Cloud Console if you haven't already. Use Python's `requests` library to send a GET request to the API endpoint, specifying the URL you want to analyze and your API key. The endpoint typically looks like: `https://www.googleapis.com/pagespeedonline/v5/runPagespeed?url=YOUR_URL&key=YOUR_API_KEY`.

Once you receive the JSON response from the API, parse it using Python's built-in `json` library. This will convert the JSON data into a Python dictionary, which makes it easier to manipulate and extract the specific data you need (e.g., performance scores, metrics, and opportunities).

Ensure MongoDB is installed and running on your local machine or server. You can download it from the official MongoDB website and follow the installation instructions for your operating system. Once installed, start the MongoDB server using the `mongod` command.

Use the `pymongo` library to establish a connection to your MongoDB instance. Install `pymongo` if you haven't already by running `pip install pymongo`. Then, create a connection to the MongoDB server using:
```python
from pymongo import MongoClient
client = MongoClient('localhost', 27017)
db = client['your_database_name']
collection = db['your_collection_name']
```

Before inserting the data into MongoDB, ensure it's structured appropriately as a dictionary or a list of dictionaries. This format is compatible with MongoDB's BSON (Binary JSON) format. Extract and organize the desired information from the parsed JSON response into this format.

Use the `insert_one` or `insert_many` methods provided by the `pymongo` library to insert the data into your MongoDB collection. For a single document, use:
```python
collection.insert_one(your_data_dictionary)
```
For multiple documents, ensure they are in a list and use:
```python
collection.insert_many(your_list_of_data_dictionaries)
```

After inserting the data, verify that it has been correctly stored in MongoDB. You can do this by querying the collection and printing the results:
```python
for document in collection.find():
print(document)
```
This step ensures that your data has been successfully transferred from Google PageSpeed Insights to MongoDB.

By following these steps, you can manually transfer data from Google PageSpeed Insights to MongoDB without the need for third-party tools or integrations.