Use the arrows to remove all entries apart from ONE of the elements layers from the list on the right-hand side. Click 'next'.
The next four screens allow you to set a variety of (mainly aesthetic) options for the legend.
Once added, the legend can be dragged to an appropriate place.
If you want to edit the legend further, you can convert it to an editable form by right-clicking it and selecting 'convert to graphics', then right-clicking again and choosing 'ungroup'. There are some basic graphic and text editing tools along the bottom of the screen that can also be used to add extra labels and annotation.