How to add a slide on Front Page
You can easily create your slides for your website on the front page. Create a post to enter slide title and slide contents in the content box. Create a new category to enter a category name from the right side.
After that, you upload your slide image from Featured Image settings bottom of categories and Save these contents to click on blue Publish button.
Please follow the bottom screenshot picture.
Show this post in Big slider Section on Front Page
Go to Appearance >> Customize >> FrontPage Sections >> Big Slider Section and find Category settings and select the category that you have created for the slider and now click on top blue Publish button to save this category.
Follow bellow screenshot image. It will helpful for yours.