It's super-easy. First, you need to add a game to your cart. You can do it in two places:
- the catalog page, by clicking the product's price, or
- a product page (for example this one), by clicking the "Add to Cart" button.
Once you've got one or more items in your cart, click the cart icon in the upper-right corner of the screen, and click "checkout now". You can also do this from the product page - "checkout now" will replace the "add to cart" button here.
Once in checkout, you need to select a payment method (credit card, PayPal etc.) and, once all data is entered, click "pay for your order now".