Converting static HTML pages to WordPress Theme

I am a student attempting to learn how to convert hardcoded pages into a WordPress theme. I have my index page running smoothly, that is not an issue. However, I cannot seem to figure out how to add the rest of my pages in. I have created a wordpress navigation, but I don’t know where to put my other pages and how to link them through WordPress. I have tried putting my second page, portfolio.php, into the folder where my index.php is and linking to it, but that doesn’t work.

I have been searching for hours to find a tutorial that goes over how to add my other static html pages, but every tutorial I have found only talks about how to add the index. Any help would be deeply appreciated.