If you have the integration set up correctly, you add an assignment and in the assignment type field you specify external tool.
I find it a little confusing that the next step is to click the Find button to the right of the link field...I felt like "what - do I have to type in the flipgrid URL?"...instead I clicked on the Find button and saw the Flipgrid option.
that linked over to flipgrid where I created a new topic. If you have some good Flipgrid experience you may get fancy as you edit that topic in the Flipgrid app...but I noted it says you can only edit that topic via Canvas. interesting.
I noted as you create the assignment, the integration establishes a new grid with the course name and a brand new topic on that grid within Flipgrid. I was coming to the questions here to look for answers as how the connection would be handled in the following circumstances:
if I had multiple cross-listed sections...will all sections go to the same topic?
if the course is duplicated within canvas?