To begin, sign up for a Pexels account and navigate to the Pexels API section. Here, you'll need to create a new API key. This key will be required to authenticate your requests to the API. Make sure to store this key securely.

Open your Google Sheet and navigate to the Extensions menu. Select "Apps Script"� to open the Google Apps Script editor. This built-in script editor will allow you to write custom JavaScript code to fetch data from the Pexels API and insert it into your Google Sheet.

In the Google Apps Script editor, write a JavaScript function to make an HTTP GET request to the Pexels API. Use the `UrlFetchApp.fetch()` method to perform this request. Ensure you include your API key in the request headers for authentication. Parse the JSON response to extract the necessary data fields you want to add to your Google Sheet.

Example code snippet:
```javascript
function fetchPexelsData() {
const apiKey = 'YOUR_PEXELS_API_KEY';
const url = 'https://api.pexels.com/v1/curated?per_page=10';
const headers = {
"Authorization": apiKey
};
const response = UrlFetchApp.fetch(url, {headers: headers});
const data = JSON.parse(response.getContentText());
return data.photos; // Adjust this line based on your data needs
}
```

In your Google Sheet, create headers in the first row to match the data fields you plan to import from Pexels. For example, if you're importing photo URLs and photographer names, create headers like "Photo URL" and "Photographer Name".

Extend your Apps Script to include a function that writes the fetched data into your Google Sheet. Loop through the data array returned by the Pexels API, and use `sheet.getRange().setValue()` or `sheet.getRange().setValues()` to populate the cells accordingly.

Example insertion code:
```javascript
function insertDataIntoSheet() {
const sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
const photos = fetchPexelsData();
photos.forEach((photo, index) => {
sheet.getRange(index + 2, 1).setValue(photo.url); // Photo URL
sheet.getRange(index + 2, 2).setValue(photo.photographer); // Photographer Name
});
}
```

To keep your data up-to-date, set a trigger to run your data-fetching script at regular intervals. In the Apps Script editor, go to the Triggers section and create a new trigger for your script. Set it to run daily, hourly, or at any preferred interval.

Before relying on your script for regular data updates, test it thoroughly. Run the script manually in the Apps Script editor to ensure it fetches and inserts data as expected. Check the console logs for any errors and adjust your code accordingly. Once satisfied, let the automated triggers handle data updates.

By following these steps, you can effectively transfer data from the Pexels API to Google Sheets without third-party tools, ensuring a direct and customized solution for your needs.