To withdraw Flare tokens from the Kraken exchange you first need to login to your Kraken account.
Press the asset that you want to transfer, this works for Flare, Songbird and many other assets.
Press the "..." button in the upper right corner.
Press the Withdraw row.
Select the amount you want to transfer, and if you haven't transfered to your Bifrost Wallet address before, you need to add your address for the token you want to transfer. Learn how to find your Flare (FLR) address.
After you have added your address, you finalize you transaction by pressing Review and finally, swipe to confirm the transaction.