You have to get it while it's free. In general that means from Friday morning through some time Sunday night. If you find an old Free Friday article and click on the book link and see a price, it's no longer free
Real free books will show the word "FREE" with a button that says "Download Now".
Books that are not free will show a price and a button that says "Buy Now".
In short, if you click on a button that says "Buy Now" you're going to pay for it.