To withdraw Flare tokens from the Coinbase exchange you first need to login to your Coinbase account.
Press Send & Receive in the header.
In the pop-up window, you choose Send. Choose the Asset you want to transfer, and paste in your Bifrost Wallet address in the To row. Learn how to find your address in Bifrost Wallet.
Verify that the token amount and the address you send to is correct. Also make sure that the Network is correct, for some tokens, you have the option to change the network, so make sure you transfer it on a network that is supported by the Bifrost Wallet. Press the Send Now button when you have confirmed the information.