In Drive, select the file you'd like to encrypt. Click the "More actions" icon in the toolbar and select "Encrypt" from the dropdown menu. You'll know your file is encrypted because a lock will appear on your file. To open an encrypted file, double-click on the file, enter your Guard security password and click "OK".
Tip: when uploading new files to Drive you can select "File (encrypted)" to automatically encrypt the file when uploading.