Customize Pro Sites Signup Page

I have read several different posts here and have found several (some broken) links to tutorials that don’t seem to work. I want to modify the contents and layout (html) of the Signup page (wp-signup.php). I have tried creating a Page Template, which worked, until I processed the form. Then, instead of being on ‘my-custom-page’ I get reverted to wp-signup.php again which has some semblance of my custom theme but there are errors due to the fact that there are hardcoded div tags with classes and IDs that I don’t use. I have seen others who copied the file to their theme folder and changed the links to the included files, but that just throws errors for me. I’m not sure what’s going on but any help is appreciated!