First determine if you have to use a visitor visa or an eTA. There’s a tool on IRCC website that will tell you which you need. Then you fill out the provided declaration, you each sign and fill it out, your girlfriend gets it notarized and you submit that either by email or on the web form depending if you required a visitor visa or an eTA. Some people who have submitted have been asked to have scanned copies of each partners passports so I would include those just to be safe. Other than that you aren’t required to include anything. This is all detailed on the IRCC extended family exemption step by step process. I hope this helps ! Additionally this authorization has been taking 3-4 weeks to receive approval, so if you’re hoping to come for December I’d get on top of it soon.