How to Create a Shortcode for Instagram Slider Widget
1. First of all, please ensure that you've installed and activated the Instagram Slider Widget plugin.
2. Navigate to "Appearance > Widgets" and drag "Instagram Slider" to the "Instagram Slider - Shortcode Generator" area.
3. After you're done with your settings, click Save and you'll see the shortcode over there:
4. Once you get your shortcode, just go to "Appearance > Customize > 9. Footer > 9.5. Instagram Slider Widget" and place it into the shortcode field. That's all.
By the way, your widget settings should be like that:
Number of images to show: 8
Template: Thumbnails - Without Border
Number of Columns: 8
Image format: Standard - 640x640