To create your own slides for the default slider all you need to do is create a new post where the Featured Image field (located in the right sidebar of the post) is the slide image. Any text you enter into the WYSIWYG editor will appear over the slide. So if you leave this empty you will just see an image.
Once you’ve added an image and possibly some text assign it to a new category such as “Slider”. This category is used to filter out the slide-specific posts from the blog posts.
Then you need to go to the Theme options page (Appearance > Customize) and click the “Slider” option. This is where you can select the category of the posts that you created to use for slide posts (eg. Slider). This will tell the slider from which categories it should select it’s posts, as well as exclude all posts in this category from the blog. If you wish to include slides that from multiple categories you can select multiple categories by holding down the Ctrl button when you select the categories.