*NOTE* It is important to upload the file EXACTLY as received from Bittrex. Do not open the file and use the "save" or "save as" function on your computer as this can effect the encoding of the file.
Login To Your Bittrex Acount
On the top right of the menu header, select the "Orders" button.
Under the "Recent Orders" section, select "Download Complete History".
In the popup, show that you aren't a robot and select the "Download" button.
Upload the downloaded file to CryptoTrader.Tax under the Bittrex tab in the Trades section (Step 2).
Once the file is populated, click "Import Trades"
You can view uploaded files by clicking the "View Uploaded Files" button: