There are four custom shape types that you can draw on your map:
In the topbar you will see a button for each of the shapes that you can draw. Press it to enter drawing mode and see the instructions on the right. When you finish drawing a shape you will automatically go back into edit mode.
When you left-click a shape to select it, it becomes editable as well. You can drag the shape to move it around or (if the shape supports it) you can move the control points to further customize it.
You have the option to change the default settings for the custom shapes. If you go to Map Settings -> Defaults you can set a new default for most shape properties. The next shapes you create will have those defaults.