Highlight horizontal edges of floors, and click to place a 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 meet at corners, they miter.
To finish the current slab edge, click Modify | Place Slab Edge tabPlacement panel Restart Slab Edge.
To start another slab edge, move the cursor to a new edge and click to place.
To finish placing slab edges, click Modify | Place Slab Edge tabSelect panelModify.
Slab added to floor
You can place slab edges in 2D views (such as plan or section views) or in 3D views. Watch the status bar to look for valid references. For example, if you are placing a slab edge on a floor, the status bar might display: Floors : Basic Floor : Reference.
When placing a slab edge in section, place the cursor near the corners of the floor to highlight the reference.