This plugin add custom fields for standard and custom post types in WordPress.
A: After installing and activating plugin you will see new menu option Settings > Just Custom Fields
A: Plugin is loaded only in Backend and create all objects only on Post edit pages and on the Settings page. So it can't affect the site.
A: first of all you need to create class for this field. You class should be extended from Just_Field main class! You can copy class /[plugins-folder]/just-custom-fields/components/input-text.php to your plugin or theme and correct it: - change name - changle class methods to use your data. Class structure is very similar to WordPress Widget classes.
Then you need to include your new component file.
And the last step: - add new hook action "add_action('jcf_register_fields', 'my_register_fields_function')" - create hook action function and call there: jcf_field_register('YOUR_COMPONENT_CLASS_NAME');