Fixing Custom Fields Missing in WordPress

This is not my explanation, but since I ran into the problem and it took a bit of time to find the correct instructions, I want to call attention to HOW TO FIX CUSTOM FIELDS NOT SHOWING IN WORDPRESS. This fixed the problem for me.

Note that I’m already using a child theme in order to add CSS and a few functions to functions.php, so remember that if you edit your theme’s functions.php without first creating a child theme, the next theme update will kill it.