Visit Opensea pro from your browser.
Click on the connect wallet icon in the top right corner > Select “Phantom” from the list of wallet providers.
You will receive a wallet connect pop up from Phantom, click on "Connect"
If you are on mobile you will need to visit https://pro.opensea.io/ in the in-app browser.
You should receive a pop-up to connect automatically, if not don't worry, click on the three lines in the top left and then click on Connect wallet.
Select "Phantom" from the list of wallet providers and then click on "Connect"
You are now connected to Opensea pro and free to make bids, purchase NFTs, or list your NFTs for sale!