The next step is to define a [ gallery ] shortcode that will be inserted into ShowPosts post. You should have the Slider Post editor open.
Click the Add Media button at the top of the editor window. This will open the Media Library page.
Next click the Create Gallery selection at the top left. This opens a window with all your Media Library images. Click to select the images you want in the slide show. You can sort image order now at the bottom of the Create Gallery window, or any time later from the post editor.
When you’re done picking your images, click the Create a new Gallery button.