I would start by asking questions! See if you can find someone from the culture and look it up! The internet is always a good place for some facts and information, but if you can get to a library, even better.
Split the culture into multiple areas you’d like to research: food, religion, clothing, politeness/rudeness/manners, legends, ghost stories, important locations, language, idioms…
The more you know about these, the better. Try and think outside of th box. People forget to check things like… if the same things are polite and rude in the culture. Like, in my culture, it’s rude to give someone shoes as a gift!
When it comes to language, don’t just google what the language is, google things like the grammar. See if it’s fairly similar to English or very different. Language has a massive impact on the way we think!
I think a good measure of if you’ve done enough research is if you’ve only used about 10% of your research as obvious references or examples in your story. The rest should be there to help you frame your point of view!