Before you create your polygon model change some of the default settings to better work through this lesson.
This displays border edges prominently on polygon meshes. Displaying border edges distinctively lets you differentiate them from other edges on your models and troubleshoot issues.
The Whole face setting lets you select polygon faces by clicking anywhere on the face (the Center setting requires you to click on the face’s center dot). In addition, when a face is selected in the scene, the entire face appears highlighted.