Update: As of 4/15/2020 If you have margin trades in your file the platform will not be able to handle them at this time. Margin trading features for Kraken are currently under development and they are coming soon!
Login To Your Kraken Account
Click on the History link on the top of the page and then click the Export tab.
Select Trades from the type drop down, select your export date range, and finally click the Select All checkbox to generate your trade history file.
Kraken will give you a trades.zip file by default. Unzip that file so you have the .csv that is inside.
Navigate to the Kraken tab on CryptoTrader.Tax and drag the file CSV file you just unzipped into the blue box.