Step 1
Click "Manage Displays" at the top left of your screen.

You can create a Layout if you have not already, by clicking Add layout

Highlighted in green at the bottom right of the screenshot below:
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Step 2
Select a Layout. Then you can open the “My Content” tab in the left sidebar and simply drag your images and videos into the Layout preview window to add them to the playlist.

Step 3
Or you can drag your images and videos from your desktop directly to the preview window to add them. Click Save and Publish when you're ready.