You apply slab edges by selecting horizontal edges of structural floors. You can also select model lines.
Structure panel
Floor drop-down
(Floor: Slab Edge). You can also access this tool by clicking Structure tab
Foundation panel
Slab drop-down
(Floor: Slab Edge).
You can also click model lines. As you click edges, Revit treats this as one continuous slab edge. If the slab edge segments are created in the same placement session and meet at corners, they miter.
Placement panel
Restart Slab Edge. 
Slab edge added to a structural floor
Tips
After placing slab edges, you can modify them in several ways.