Access the Site menu → Global Configuration → Text Filters and for Super Users make sure the No Filtering option is enabled.
In the Extensions tab go to Plugin Manager → Editor TinyMCE → Basic Options → New Lines: Br Elements → Prohibited Elements. Make sure you remove Script and iFrame from that list.
While still in Extensions, access Template Manager → Templates and choose your active template. Click on its name to edit it.
Next to $doc → addStyleSheet, insert this line: http://www.123contactform.com/includes/tinybox/tinyboxstyle.css
Make sure the line is added within the tags of the file.
(Optional: if you don’t have $doc = JFactory::getDocument(); declared already, add it before $doc->addStyleSheet).
Now go to the article you want to insert forms to. In Edit Article click on Source Code (or Tools → Source Code).
In your 123ContactForm dashboard, access the form menu, Get Code and Publish → Lightbox. Choose the option you need (text link, image, floating button or auto popup), then copy the form code and paste it into the Joomla article source code box. Save the content and you’re done.