On a product's detail page, tap Buy Now.
You are taken straight to checkout with the item, a quantity of 1, and the total already filled in.
Adjust the quantity if the product allows more than one, then continue to review and pay.
Note: Buy Now skips the basket entirely, the item is not added to it. It is only available for in-stock products.
If the product goes out of stock in the moment between viewing it and tapping Buy Now, you will see "Sorry, this item is no longer available" and be returned to the product page.
FAQ
Does Buy Now affect items already in my basket?
No. A Buy Now checkout is independent of anything in your basket.
What happens after I tap Buy Now?
You move to the checkout and payment steps. See the article on completing your purchase for what to expect there.