Regular conversations in the language you want to learn. You CAN'T use your native language at all and you must respond in the language you are learning. And you can't get help from your conversation partner by having them translate to your native language. Doing it this way forces you to search your memory for the words you need and the adrenaline of having to understand the words and answer correctly helps you remember. Do this enough and you will get the language's basics in a relatively short time. It's not the easiest way to learn but it's the fastest way to get to fluency.

BTW, stay away from Duolingo. It won't get to the level you want to be at. There are better and faster ways to learn a new language.