to be able to run the checkout wizard and create the checkout pages, you will first need to have a cart display page.
you could add the Donation amount form element to the checkout page, and add the Add to Cart Server Behavior. On the server behaviors panel click the plus button and select eCart -> Add to Cart -> Add to cart, set the trigger to the checkout button being pressed.
enter the item name and description, for the price, click the lightning bolt icon and select the amount form element.