Basics: Connecting objects with lines
You can connect any two objects with the Line tool (
).
). Click the Line tool. (Note that this tool might look different if you change its style.) 
Click the first object you would like to connect. This object becomes the "source" of the line. 
Click the second object you would like to connect. This object becomes the "destination" of the line. 
Once objects are connected by a line, they stay connected no matter how much you change them or move them around. That means the logical integrity of your diagram is preserved regardless of how you choose to style it. You can disconnect a line from an object by selecting the line and then dragging one of its endpoints.