Check out this walkthrough on how to use LANDR on Symphonic Distribution's system "SymphonicMS".
Step 1: Log on and click the "LANDR" button.
Step 2: Click "Free Preview" or "Master A Track". You will be able to hear a snippet of what your master will sound like before paying. Additionally, you'll be able to preview the difference with a song by Tiga!
Step 3: Upload your file and then when your satisfied simply pay for it and you will receive the mastered file on the email address associated with your Symphonic Distribution account. Additionally, it will be available in the "Mastering History" page.