To access TMDb data, you need an API key. Visit the TMDb website, sign up for an account, and navigate to the API section. Apply for an API key by providing necessary details about your application. Once approved, TMDb will provide you with an API key to use in your requests.

Familiarize yourself with the TMDb API documentation, which details the endpoints, request formats, and response structures. Decide on the specific data you want to fetch (e.g., movie details, actor information) and note the corresponding API endpoints and parameters.

Open Google Sheets, navigate to Extensions > Apps Script. This will open the Apps Script editor. Use the following template to write a script that makes HTTP GET requests to the TMDb API:
```javascript
function fetchTMDbData() {
const apiKey = 'YOUR_TMDB_API_KEY';
const url = `https://api.themoviedb.org/3/movie/popular?api_key=${apiKey}`;
const response = UrlFetchApp.fetch(url);
const json = response.getContentText();
const data = JSON.parse(json);

return data.results; // Modify this according to the specific data structure
}
```

In your script, parse the JSON response from the TMDb API to extract the data you need. For example, if you're interested in movie titles and release dates, loop through the `results` array and extract these fields:
```javascript
function parseData() {
const movies = fetchTMDbData();
const parsedData = movies.map(movie => [movie.title, movie.release_date]);
return parsedData;
}
```

Use the Google Sheets API in Apps Script to write the parsed data into your spreadsheet. Identify the range where you want to place the data:
```javascript
function writeToSheet() {
const sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
const parsedData = parseData();
sheet.getRange(2, 1, parsedData.length, parsedData[0].length).setValues(parsedData);
}
```

To keep your data updated, automate the script execution using triggers. In the Apps Script editor, go to Triggers (the clock icon), and set up a time-driven trigger to run the `writeToSheet` function periodically, such as daily or weekly.

Run your script manually to ensure it works as expected. Check the Google Sheets to verify that the data appears correctly. If there are errors, use the Logger in Apps Script to debug:
```javascript
function fetchTMDbData() {
// existing code...
Logger.log(data);
}
```
Use `Logger.log()` to output intermediate results for debugging.

By following these steps, you can transfer data from TMDb into Google Sheets without relying on third-party connectors or integrations.