is there a simple step by step tutorial on adding a Checkbox list

You can use this tutorial:
https://docs.piklist.com/getting-started/

With two changes:
1) Instead of putting your code in the /meta-boxes folder, put it in the /terms folder.

2) Change Post Type: post in the comment block to Taxonomy: your-taxonomy-name

Then use this code to add the checkbox:
https://docs.piklist.com/fields/types/checkbox/

Let us know if you still need help

What is the difference between Folder Structure for your theme and Folder Structure for your plugin? Is the ‘Folder Structure for your theme’ the front-end, and the ‘Folder Structure for your plugin’ the back-end/admin section of WP?

Also, my folder structure looks different: wp-content/plugins/piklist/parts and there is no /terms folder. I see the piklist-demo /wp-content/plugins/piklist/add-ons/piklist-demos/parts/terms has a /terms folder with lots of files in it.

I expanded on the explanation here:
https://docs.piklist.com/getting-started/#folder-structure-for-your-theme

Let me know if that helps.

how can I get a meta-box to visually group different taxonomies (selects)? Somehow I can’t get anything to show if set in the terms folder.

If you want the select field to appear when you’re creating Posts or a Post Type, then your code needs to be in the meta-boxes/ folder.

You can use this code:
https://docs.piklist.com/tutorials/taxonomies-terms/display-taxonomies-radio-checkboxes-select-list/

I have been looking at this, but somehow can’t get it to work. Would I still need to add code to my plug-in.php file?

I should mention … this is me you have been helping (same topic)