I take it you're proficient in HTML, but not PHP? After the user fills in the forms on the web page he/she would submit the form. This would go to a PHP page that would create a new HTML document using the information submitted by the user. This document would then be fed to DOMPDF to convert to PDF.
There are a
lot of tutorials out there about how to use PHP to capture form data and create a HTML document. Find one you like and see how far you can get. You should focus on creating the HTML document from the form submitted by the user. That's probably the hard part. Because creating a PDF after that is as simple as:
$dompdf = new DOMPDF();
$dompdf->load_html($html);
$dompdf->render();
$dompdf->stream('document.pdf');