The fields on the payment form like credit card, expiration date, ccv2 code, etc are already validated. Credit card number is validated using a mod 10 check. If you open up the page you want to add the validation to, for example if you want to add validation to the phone field on the step 1 page open up step1.ascx located in \desktopmodules\smith.buynow and scroll down to the control you want to add validation to and add the following validation control:
<asp:RegularExpressionValidator ID="rfvPhone1" ValidationExpression="^([0-9\(\)\/\+ \-]*)$" runat="server" ErrorMessage="Enter a valid phone"/>
You can see its already there in the page and just commented out. We had added validation on the fields phone, postal code, etc but had to comment it out becuase it wasnt working for the all the possible variations around the world and people get fruistrated when they enter something and they get a message to enter in the proper format.