2. Navigate to Fossa Guard in Chrome Web Store and click 'Add to Opera'
3. Agree with the warning. Click 'Install' on the extension info screen and 'Yes, Install' on the next screen.
4. 'Login into Extension' by choosing the appropriate Gmail account
5. Navigate to Gmail - you have got S/MIME compose button and ability to send and to view S/MIME emails.
Please note that you should use the same Google account in Gmail and Fossa Guard extensions