Missionaries come allready traumatized. They bring a list of no-can-do phrases. Stuff they should avoid so that they don't scare people off. A mentality of "You can do no mistakes".
Well most no-can-do phrases are crap anyway. Missionaries come here with their best clothes because someone tells them that we use fancy clothes in Portugal. No jeans, no havannas, only suits. We have a bunch of missionaries from Canada that don't use more than half the clothes they brought because someone told them we used fancy clothes all the time! Ridiculous. And this is just an example.
The result is a plastic life. And it notices, too. You know when you meet someone, and you feel like he is hiding something? That he only tells nice things? Everything is ok, everything is fine, they never say no. Well, that's how a missionary looks like when he gets to a country. A friendly robot, with no identity.
If you really want to learn the culture of a country, you have to talk with people from that country. And relax, at least where globalization has reached, people understand that you are different. They will be able to tolerate some mistakes because you are foreign. Just be humble and honest. People like honesty. They prefer it to plastic people that are allways smilling and that look like everything goes well with them.
And another thing. You are foreign. You are different. People will notice that even if you stay in a country for 30 years. There's no point in trying to imitate locals, you'll never look or sound like them. Just relax, being different is not that bad. And if you don't like eating snails (we do in Portugal), that's ok. You'll still be accepted, people will still hear your message.
If you want a recipe: Learn the language, do your best to learn the culture humbly. But that will never be your culture, and people know that. Just be polite when you say no. Be humble. Be yourself, but in a humbly polite way. Create relationships. Love people. That should be enough.