After you've configured your plan, you'll see a similar screen that has a "Checkout" button on the top left. Once you've confirmed all order details, changed the quantity to the amount of servers you need, and entered any promotional codes you would like to use, click the button.
Next, you'll be taken to the checkout screen. If you have a Google account, you can click "Sign up with Google" however we have highlighted everything you must enter if you do not use Google. Once you have filled everything out, click the big blue "Complete Order" button to pay! You can follow our guide on Payment if you need help.