Yeah you were doing it right. Once you have funds in your MetaMask then you can click the buy and it will pop up with the amount to pay plus the gas fee. The gas fee determines how quick your transaction will confirm but I usually just do the minimum. From there the website may tell you that it failed because it will wait for the tx to confirm but once it goes through it will process the purchase and put it in your wallet. The only thing that might cause your tx to not go through is if someone else buys the fighter before you get them.