Navigate to the NFT you want to sell and click on Sell in the top-right corner. If you don't see the Sell button, make sure you're connected to the correct wallet.
Next, you'll see a modal appear. Enter the price you want to list your NFT at, and click List Item.
If this is your first time selling an item from this collection, the site will ask for you to approve the collection for trading. Click Approve.
You only need to do this once per collection. Once you've approved a collection, you don't need to do it again, unless you revoke the approval later on.
Lastly, you'll need to confirm the listing with a transaction from your wallet.