When you make a character that isn't your own race or ethnic background, you have to research things, in order to avoid misrepresentation. But, what must you research about a given ethnic group? How much information is required as a base minimum?
I'd like to add a few more questions to @ggiaquin's answer, because how much and how little research does depend on a lot of things, and you can never ask enough questions to get to the bottom of what exactly you need.
Is your character a main or secondary character? If it's the former, greater research is obviously required.
Is the character living within a comunnity of their own ethnic background or with a community of a different ethnic background? If the latter, is it sensitive or even hostile to the character's ethny, or does the community simply ignore it? Do keep in mind that if it's the first case, most people will live their traditions as something natural and won't even notice them. If it's the second case, most likely there are traditions the character will miss (even if you boil it down to missing one ingredient that isn't easy to find at supermarkets).
Is your character a traditionalist (attached to their cultural traditions), a modernist (forget traditions, follow the most up-to-date trends) or something in-between (a British person drinking tea compulsively even though they frown upon anything that is conservative and 'British' as old-fashioned)? If it's the second, forget most of the research and treat them as a regular westerner (I'm assuming this tale is set in the western world). If it's the first case, do a very detailed research.
Word of caution:
Avoid sticking to stereotypes.
I'm extremely fond of my own ethnic traditions, from music to food. Nevertheless, there is some typical food stuff I hate. For instance, olives and grilled sardine. Every Portuguese likes olives and grilled sardines (stereotype here). I don't like them. I know quite a lot of people who also don't like one or the other. Anyway, my father will sometimes say 'you're not Portuguese' (in jest) because of it. Think if your character has a dislike for something typical and decide if their family accept it well (in jest, like my father) or if they insist that the character must like it and therefore criticise them relentlessly (maybe even with insults).
Moreover, decide if the character would like to like something they don't. For instance, I love the smell of sardine and to see a grilled sardine dripping its juicy fat onto the bread... I really wish I liked it. It looks delicious! My stomach still insists in disagreeing with my eyes and nose though. Or perhaps the character feels left out, like they can't fit in, due to that 'flaw'.
Anyway, even if you choose to have a traditionalist, remember that traditions are felt and lived differently from place to place and even from family to family. It's part of the Portuguese tradition to eat boiled cod and cabbage on Christmas Eve. I know some very few who also add chickpeas. I know some who, even though they follow every other tradition, avoid the cod because a child (or themselves) doesn't like it. Or they have two dishes: cod for the ones who want it and something else for who doesn't like cod.
The easiest thing will probably be to talk to someone from the ethnic background you want (write down a long list of everyday stuff to ask, if need be), though they'll probably look at you funny about stuff that they find obvious so why are you even asking about it. Like bacon. In Portugal the word 'bacon' refers to something that has been thoroughly cured through smoking so it doesn't require cooking. When I told an American friend I loved bacon sandwiches, he asked how I cooked it. I made a face and said 'I don't cook it. No one does.' He looked at me like I was an alien: 'you eat bacon raw?' I couldn't get his horror until he showed me what he calls 'bacon'.
If you can't talk to anyone, then I suggest an approach in three steps:
Read about traditional food, music, literature, etc. If possible, experience it yourself.
Read blogs (or books or articles) written by people of the required ethnic background while they were living elsewhere. I've read some very funny stuff written by students in foreign countries (even if the language is the same). I also remember one where the student had just gone to a 'foreign' state. Very eye-opening.
Once you got a feel for the ethnic cultural background you chose, decide which traditions your character will like, dislike or not care about. Or, inversely, just pick one or two traditions the character adheres to, consciously or unconsciously.
Note: I'm using the term 'tradition' very, very losely in order to encompass almost anything that is typical to a community.
Depends on what you want to do with their character? Is it modern day? Post modern? something from the past? Does this character eat at all in your story? What type of things would this character do that would need to have cultural influence? Are they Asian doing a kung fu tournament with family and cultural influence/pressure to do so?
What is the setting? Location? Is this inner city New York where an Afican-American teenager will mostly be playing basketball? Or is this in the streets of Brazil where that same teenager is now playing soccer?
Ultimately, you need to think about what culture specific topics may be needed. If your character is going to have implied meals (whereas you specifically write about the character eating), looking up something like what do African tribal people eat for breakfast won't help the story.
The "How" much will be enough is up to you on how much does the culture of the character play in the story. They may just be a hispanic male from Mexico but otherwise do everything a Caucasian does and doesn't eat Mexican food but instead enjoys a good burger. Heck, I have Italian ancestry and I don't like Italian food!
Nothing. Everything. In the end, fiction is not about what you have researched, it is about what you have lived. Of course, writers of historicals or space operas have not actually lived in those environments, but thematically and in terms of their characters, they are a reflection of lived experience. I am not sure you can get the essence and the authenticity of a character from research.