I’m reading this thread and feeling for you! I went through a lot of this initially and here is what I can tell you at a high level. You need to install Ruby. Once you accomplish that step, you really won’t have to think about it any more. You will code your project with CSS (or SASS/SCSS), and HTML. You might use a few plugins and you will need to familiarize yourself with the super simple Liquid language (Shopify uses it for people designing stores).
After you follow the setup instructions you can create your site locally and then publish to a hosting provider. I actually use GitHub Pages, which is free so you and your customer can immediately start reviewing the site.
I left Wordpress and moved to Jekyll and it was hard. You install Wordpress as an app. Jekyll is more of a framework. You have to build the site from scratch (or find themes) and add program what you want. You can always start with the default Minima theme that Jekyll ships with and modify it.
After your dev machine is set up locally, you can check out my videos here and they will get you up and running:
From there, if you want to keep learning you can link to my YouTube channel and find tons of content I created during my journey setting up Jekyll for my site. Of course this Jekyll Talk forum and super helpful.
Hope this helps as a more generic overview and did not interrupt the discussion too much.