Click on 'Order/Cart Settings'.
Scroll down to 'Checkout Process Mode:' and Choose 'OPC'.
Hit 'Save changes' at the bottom of the page.
*Note that enabling OPC does not remove mandatory user registration. To disable forced registration, you will need to scroll up to 'Allow Create Account' and set it to 'No' . This however will not keep users from registering if they want to, it just takes it away from the checkout process.
