To make a playlist, select the Playlist tab then click Add. Choose a name and click Create.
To add content to this playlist, (with the appropriate playlist selected on the left) Click Add above the preview panel.
When you add any number of items to the playlist you can modify how those items will display; including the type of transition, display duration, and how often they appear.
Items can be deleted from the playlist or replaced from the gallery media at any time in much the same way as in the Gallery.
You can also add a Sub-playlist with the Add button in much the same way as you added items from the gallery. This allows you to choose another playlist you created to run as it exists as a single item.