By listening more. It takes a lot of time listening to develop an ear for a foreign language.
When I learned Portuguese and finally got my ear for Portuguese, I'd been living in Portugal for six months working 70 hours a week with non-English speakers speaking only Portuguese before I got my ear for it.
When I learned Spanish, though, it didn't take me nearly as long. I got my ear in about three months living in Spain, going to university, and not interacting with anyone in English ever, only Spanish.
When you get your ear for a language, it happens overnight. It's like something clicks in your brain and you suddenly easily understand and aren't struggling to catch 40% or 50% or whatever percent anymore.
The best and fastest way to get an ear is through immersion. So if you want to get an ear for American English, you're best and fastest way to do that is to live in America, interact with English-speaking Americans, and be very disciplined about only speaking and interacting in English, never your native language.
Beyond that, watching American television and movies is a good way to get an ear for American English. People's language comprehension vocabulary is more than 20 times what people's speaking vocabulary is.
So when I lived in Portugal, I got my ear for Brazilian Portuguese, which is vastly different than European Portuguese, by watching Brazilian telenovelas (i.e., soap operas) on TV. For the first few, I understood very little, but then suddenly about the third one in, I got my ear for it and started understanding it all.
Likewise, when I lived in Florida, I worked with many, many people who couldn't speak English at all, but it was extremely common for them to watch American television and understand everything because they'd gain an ear for it through watching American television enough that they just understood, even though they couldn't speak it at all.
It was also very common for non-English speaking parents with English-speaking kids to get an ear for it. All the time I witnessed conversations where kids who spoke to their parents in English and their parents spoke back to them in Spanish. Both sides fully comprehended what the others were saying even though neither side spoke the other's language well enough to communicate it, especially not the parents, because they'd gained an ear for it by simply listening to it long enough.