Sitter canceled 2 days before trip, how do we leave feedback?

How do we leave feedback for a booking that was canceled by the sitter?
We just had our sitter cancel 2 days before we leave for a 9 day trip. We booked over a month ago and had set up three advance walks so our anxious dog could get to know sitter beforehand. The sitter rescheduled twice but ended up showing up for 2 of the walks and then canceled the 3rd saying he was overcommitted/overwhelmed and that he couldn't watch our dog during our trip. This is leaving us scrambling and stressed out trying to find someone to watch our dog during spring break. I would like to leave a review for this sitter.