Place the cursor where you want the line
Go to the Insert tab
Select Shapes
Choose the Line tool
Click and drag to draw the line
Hold Shift while dragging to keep the line straight
Release the mouse to place the line
Use the Shape Format tab to change the line color, weight, or style
To add a horizontal line quickly, type three hyphens and press Enter
To add a horizontal line quickly, type three underscores and press Enter
To add a horizontal line quickly, type three equal signs and press Enter