I am looking for the GST/HST quick method form to complete so I can figure out how much I owe.

can only find instructions!
I am self employed. and I forgot to pay quarterly.

Troubleshooting

You can't file a GST/HST return using TurboTax. This CRA page tells you all the ways to can file a GST/HST return: 

https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/gst-hst-businesses/complete-f...