Easy answer: Learn. I didn't know how to do it either, until I learned. How do you know to speak or write in English? It just takes practice. I believe in you Milk, I believe in you.
Did I mention that I learned how to tie my shoes around the age of 11? I'm a late bloomer... although I think I might pick up whistling later in life.
It's ok... I have been learning English for about 10 years now and I can still barely speak it without fucking up somehow.