Roxana,
you can find the VTK Getting Started Guide and How To's here.
Generally the checkout page submits to the confirm page so open the confirm page in DW and open the bindings panel and find the Form Data Entry open that up and navigate to your checkout page and select it.
You will then have the form values from the checkout page exposed on the confirm page so you can validate them but DON'T FORGET to send the purchaser back to the checkout page if validation fails and display your failure messages there.
Screenshots attached: