A quick snippet for registering custom admin columns for a post type in WordPress.

register-wordpress-admin-columns.php

<?php

$post_type = 'my-post-type';

add_filter( 'manage_' . $post_type . '_posts_columns', function ( $defaults ) {
	$defaults['custom-one'] = 'One';
	$defaults['custom-two'] = 'Two';

	return $defaults;
} );

add_action( 'manage_' . $post_type . '_posts_custom_column', function ( $column_name, $post_id ) {
	if ( $column_name == 'custom-one' ) {
		echo 'TODO';
	}
	if ( $column_name == 'custom-two' ) {
		echo 'TODO';
	}
}, 10, 2 );