Use your mouse to click on the Insert button in the top menu. From the drop-down menu, click on Line. Then click the Arrow button.
Move your mouse pointer over a shape or text box. Purple circles will appear around the shape.
Place your mouse pointer directly over one of the purple circles. Then click and drag your mouse pointer over to a different purple circle on a different shape. If you get your mouse pointer close to the second purple circle, the line should snap into place.
When you release the mouse button, the line will connect the two shapes.
You can make sure by moving the shapes and seeing if the line also moves.