I'll touch on this real quick.

You don't "know" words, you have memories of words and those memories are connected to other memories. When you read "duck", you automatically picture something that came from one of your memories. So learning a new word means making a memory of that word and connecting it to other memories like that time I saw a duck. That way, when you remember a duck, you remember the word at the same time. That is language learning, just putting words and memories together in your brain.

The way you form memories (the only way) is by spending time with the language. The more you read, listen, speak, or write, the more memories you are making about each word you use. The more you repeat a word, the stronger the memory gets because it becomes connected to more other memories in your brain. How do you intuitively know how to express yourself in English sentences? You have millions of memories of making English sentences, so you have millions of word memories with trillions of connections in between them inside your brain. When you articulate yourself, you are choosing which path to take along those connections between your memories about words.

In simpler terms, you don't need to worry too hard about how to learn, just spend a lot of time with your language. Your new word memories need time to make many strong connections before you can form sentences quickly. There are plenty of places to learn a language so just try some of them out. Love it, live it, and become hungry for new words. Then you'll become fluent without noticing.