Custom Contact Form/Processing Issue

I’m developing a WP theme and have included a contact page. The php file has the input fields, etc., and it’s saved as a WP page. I know I could easily use a plug-in for a contact form, but in this case I want to do it manually. I already have contact form code that I know works outside of WP, but I’d like to integrate it into the new page. It consists of two files. The first is the one I made into a page, which uses JS validation, as well as server-side validation. The action is set to post back to the page itself, but there’s code at the top of the page to change the header location to a “contact-confirm.php”, based on a processing indicator and if the page passed validation. The issue I’m having currently with WordPress is that the page will just post back to itself, regardless of whether validation was “passed” or not. What do I need to change in order for these pages to function properly? (ie: For the form, what would go inside action=””? And for “header(‘location:’…)?)

(I am using permalinks, though I’m not sure if that would effect anything.)

I really appreciate any information/suggestions anyone has. Thanks in advance!