1. Click on stockpile to make it editable.
2. Click it again to save, this will bring up the Stockpile Saving Menu
3. Click "Add floor points"
4. Click on the floor next to the stockpile setting at least 5 points representative of the floor
   1. The points should be good for projecting a plane, so they should not be exactly aligned. Think of a three-legged stool, if the legs form a triangle, the stool is stable. If the legs were all set to form a line, the stool wouldn't be very useful. The same concept applies when you set floor points.
   2. Think of a table and its legs, if the point where all the legs touch the ground are aligned, the table cannot stay up. If the legs are properly spread out in North-South and East-West directions, the table is stable.
5. Press "Enter" when finished setting floor
6. View stockpile in 3D to validate floor is set correctly