Use this tool to trace the edges of the object you want to extract. You may need to adjust the brush size of the tool – you can do this in the Tool Options section on the right side of the window after you select the Edge Marker.
Note that you don't have to outline the object in one fell swoop. You can take your time, starting and stopping your edge tracing as often as you like. You can also select the edges of multiple items if you want to extract more than one object.
While tracing the edges with the Edge Marker, you may want to stop from time to time to use the Pan tool to reposition your image. This way, you can zoom in close while tracing and don't need the entire object in your screen view.
If you make a mistake while tracing, you can select the Edge Eraser tool to remove any markings made with the Edge Marker.