Source: https://www.sitepoint.com/adding-custom-meta-boxes-to-wordpress/

wp-add-custom-meta-box.php

function custom_meta_box_markup()
{
  // custom meta box markup goes here
}

function add_custom_meta_box()
{
  add_meta_box("demo-meta-box", "Custom Meta Box", "custom_meta_box_markup", "post", "side", "high", null);
}

add_action("add_meta_boxes", "add_custom_meta_box");

// Source: https://www.sitepoint.com/adding-custom-meta-boxes-to-wordpress/