I believe there are two ways.
1) Preferable: As Chris mentioned, working with a native speaker, especially one who is trained in phonetics
2) If you can't afford a teacher, you can make Anki software flashcards that include sound files from native speakers. You can find sound files from almost any language on Forvo.com, among others. Also, if you learn the international phonetic alphabet and can write your word on your flashcard using that, you will know better how to form the sounds of that word with your mouth.