Hi Bidur,
In the documentation there is a broken link, that’s why we can’t setup the slider either
Step 2 – Create a Home Page Slider
You need to create a slider using widgetkit plugin. The complete tutorial to make a slideshow can be find here: https://www.styledthemes.com/flat-responsive-setup/387-create-a-slider-like-the-demo-website-using-widgetkit and place the shortcode that come from widget kit to banner-wide widget position.