Example 1
Before you travel to a foreign country, it's a good idea to brush up on the local customs.
That's some solid advice. While a thumbs up gesture means "good job" in the United States, in Singapore it's the equivalent of sticking your tongue out at somebody, or worse. If you knew that customs is the right word for this sentence because it's referring to acceptable local behaviors, then thumbs up to you (the American kind).
Example 2
Every year, Reggie wears the same Halloween costume. He's always a "cereal killer."
We can appreciate a good pun as much as the next person, but it might be time for Reggie to switch things up. In this example, costume is the right word because it refers to a special type of dress reserved for Halloween.
Example 3
Anne works at a fabric store in Santa Monica, and she says some of her best customers are Hollywood costumers.
In this double feature of an example, we have customers, which refers to the people that buy fabric from Anne, and then we have Hollywood costumers, who are people who make imaginative outfits for the big screen.