It's not hard if you understand a bit of HTML. You have a sample given on that page. Basically, you should prepare thumbnails and big images. Create a div or paragraph and assign id="gallery" to it. Inside it add thumbnail images and link bigger images with them.
You can do all that with Joomla content editor.
If you need a simpler way to create Lightbox slideshows, we have Hot Photo Gallery Plugin, so you may consider this too:
www.hotjoomlatemplates.com/store/buy-joo...allery-joomla-plugin
