You will see a link to bid on your desired city when you perform a flight search. Alternatively, you can view the entire collection on OpenSea by clicking this link.
For example, if you want to bid on New York City, then you can search for a route such as NYC to LAS. At the top of the search results page, you will see the option to bid and/or buy NYC.
Click on the "Place a Bid" button and you will be taken to an external website, OpenSea, to complete your transaction. There you can create an account if you don’t already have one. Once you are logged into your account and link your wallet, you can bid on the city.