Let's start off with a map which already has the 3D floors.
Open the map in Visual Mode. Looking at the 3D floors from above press Alt to enter the Visual Editing Mode. Moving the cursor to the edge of a 3D floor will show the edge in yellow. Clicking on the yellow edge turns to orange and lock it in. To exit this mode press C or quickly go out of and back in the Visual Mode.
In the video I try to show how it is done.
Once done with placing the slopes, the map can be further edited , for example