Open your Coinbase account and select accounts in the top center.
Select your ETH Wallet
Then Select Send. You will get a popup box that asks for the recipient address. Open your MetaMask by clicking on the Fox Icon.
Then click on the account name to copy the address into the clipboard.
Paste the address into the recipient address on Coinbase.
Select the amount you want to send, or send max then press continue.
Double check everything, then confirm your send.
If you'd like to watch the step-by-step video: