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