First, try to imagine the most beautiful sound in your head and then play the first note of your piece while still mentally producing that tone. After a while, your brain will send signals to the rest of your body that this is the tone it wants and your tone will adapt. Once you're happy with the sound, do the same with the next few notes. Remember to always focus on the tone you are making in your head.
It's a bit like doing tai chi on your instrument. You do everything in super slow motion. You don't need to play the whole piece like that. Just the first few passages. After a short while, your tone will match your mental projection quicker and you won't need to play so slow to get what you want. This will relax you and give you a nice smooth tone.