Breaking down xhtml into wordpress template help

Hi There,

At the moment, I’m trying to create a theme sourced from a xhtml/css page I’ve made.

I’ve been trying to follow the tutorial here:
https://www.andrewstrojny.com/web-design/convert-xhtml-css-to-wordpress/

But seem to keep stumbling… (this is my first go at css & wordpress)

Would someone please be able to take a look at my code and advise me on how to build my index/header/footer/sidebar php files correctly using my xhtml & css template?

My HTML & css are located here (note body & sidebar are auto height):
https://www.nelga.com/glen_asf.html
https://www.nelga.com/safedropdown.css

And all I can get at the moment is this: https://www.maxco.com.au/asf/

BTW, if you were wondering, it’s for a not-for-profit Australian science festival.
Thank you so much for your time.

Glen