Break 'ethnic' down into sounds: [ETH] + [NIK] - say it out loud and exaggerate the sounds until you can consistently produce them. Record yourself saying 'ethnic' in full sentences, then watch yourself and listen. You'll be able to mark your mistakes quite easily.

/ˈeθ·nɪk/ relating to or characteristic of a large group of people who have the same national, racial, or cultural origins, and who usually speak the same language: an ethnic neighborhood.

/ˈeθ.nɪ.kəl.i/ in a way that relates to someone's ethnicity (= the fact of belonging to a large group of people with a shared culture, language, history, set of traditions, etc.): ethnically related communities.