To add a polygon to your Polygon post, press "Replace Drawing" and start to click on the map, clicking twice on a point to finish the polygon area. You can also upload a geoJSON if you prefer.
Flat polygons are standard polygons that just cover an area on the map. There are other types of polygons available as well (3D and image).
Color
Set the color of the polygon.
Opacity
Set the transparency of the polygon.
Outline Color
The outline color is a 1 pixel border around polygons. Unfortunately the width cannot be adjusted. To make an adjustable border around your polygon, the best strategy is to make a line that has the same geography as your polygon.