you show up at the border, ask to stay for whatever period of time you plan on, and either they stamp your passport, give you a visitor's record to track your stay, or let you through without any concern.
If you haven't submited your application yet, bring proof of your intent (copy of application, receipts for fees). If you have submitted your application, they will confirm it's been received and let you in. They prefer US visitors when you can show you maintain a residence in the US. If you can work remotely for a US employer, that helps too. No one can tell you for sure what will happen at the border, because every crossing is unique, and it depends on the agent you get.
Once you are in Canada, you can apply to extend your stay as a visitor to help get you through the process (if it's even necessary).