Here's a short guide on how to add a user to the sudoers file and grant them full administrative permissions on a Debian system:

1. **Add the user to the sudo group**:
   Assuming the username is `username`, add the user to the `sudo` group. This group has sudo privileges by default.
   ```bash
   sudo usermod -aG sudo username
   ```

2. **Verify the user is added to the sudo group**:
   Check the group membership of the user to ensure they have been added to the `sudo` group.
   ```bash
   groups username
   ```

3. **Edit the sudoers file (optional)**:
   If you need to provide specific permissions or want to manually add the user to the sudoers file, use the `visudo` command to safely edit the file:
   ```bash
   sudo visudo
   ```

4. **Add user with full sudo permissions**:
   Within the `visudo` file, add the following line to grant full sudo permissions to the user `username`:
   ```bash
   username ALL=(ALL:ALL) ALL
   ```

5. **Save and exit**:
   If using the default editor (`nano`), save the file by pressing `Ctrl+O`, then press `Enter` to confirm. Exit the editor by pressing `Ctrl+X`.

6. **Test the sudo access**:
   Switch to the user and test if they have sudo privileges:
   ```bash
   su - username
   sudo ls /root
   ```

   The command should prompt for the user's password and then list the contents of the `/root` directory if the user has proper sudo access.

That's it! The user `username` should now have full administrative permissions using `sudo`.