non-plugin contact form

I am having trouble getting a contact form to work. I have used the form successfully on a static site for several years with no problem. I am now trying to add it to WP. The form is a full contact form for a local service site (Not A Feedback Form).

This is what I have done (obviously wrong):
Pasted the form into a page “contact-us”
added the form generator contact-form.php to my theme-child folder – I think this is where I have gone wrong.

I am getting 404 errors and I think it may be the location of the form generator. I have tried

<form method=”POST” action=”https://localhost/MyWebsite/contact-form.php”&gt;
and
<form method=”POST” action=”../contact-form.php”>

I also tried a couple of plugin forms but most that I looked at are feedback forms, not full contact forms. I would like to use the one I have. If I need to turn it into a plugin, that’s fine. It will give me something else to learn.

My question is what should be the correct path in the form method or, alternatively, where should contact-form.php be located?

thanks