The designs you save in your collections show a ♡ button when you hover over them with the mouse. Mark them as a favorite by clicking the outlined heart to turn it into a ♥.
Choosing favorites works both in Grid and Tablet view:
Picking a favorite in Small Grid view
Picking a favorite in Table view
Click the ♥ button (1)
in the sidebar to bring up the menu (2)
. From here you can pick to filter your collection by:
Note that filtering by favorites will affect both your sidebar and your current collection view: