How can customize Checkout Method ( Create Order from CartOr

i'm using sitefinity ecommerce custom checkout and in method PlaceOrder of Order helper , i don't know , the way to run of method Checkout
IPaymentResponse paymentResponse = ordersManager.Checkout(cartOrderId, checkoutState, customer);
Please suggest me flow chart to run Checkout
Lik Customize sitefinity ecommerce
 www.sitefinity.com/.../sitefinity_ecommerce_custom_checkout_ndash_part_1