You should not have to edit any conf files to get XAMPP up and running and doing it wrong could open a security hole that you may regret.
You must not be able to access any XAMPP files and folders from the Internet only your own web site files and folders, so the tutorial is opening a security hole which is very wrong indeed.
So please return your httpd-xampp.conf file back to the default New XAMPP security concept settings before proceeding.
With regard to your tutorial I have no idea what that is as you failed to give details or provide a link to it.
First, make sure that your installation works by Starting Apache and MySQL in the XAMPP Control Panel making sure they both show a green running indication then type in your browser:http://localhost/xampp/
You should see the Language setting page and then the XAMPP Welcome Demo page if your installation is working fine.
Now delete the index.php and the index.html file already in the htdocs folder and replace it with your own web site index file.
When the Apache server is running and you have put your web site files and folders in the htdocs folder then when you type in your no-ip address Apache should find the index file of your web site.