As had been said - practice. It’s how native speakers of a language get good at it.
You can improve your vocabulary, comprehension and accent by watching TV shows and movies (I remember meeting Dutch kids on holiday who I thought were American as they learn so much from the screen), and grammar and the like can come from reading. However, that ease of conversation and the speed of your recall of words just comes from talking a lot. Try finding an intermediate to advanced language class where they insist on people talking in that language all the time.