How do I define border edges?

To define border edges first get into the edit mode and click on the pencil icon. Next click on the style tab and choose the border icon. Finally choose which edges and click on the edge buttons. 

Here is how to define border edges:

Step 2: Click on the edge icon you want (A).
You will find four icons denoting all four edges or sides (arranged in order Top-Bottom-Left-Right), 
Step 3: Choose width (B) and color (C) and click done.