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
Release the mouse to place the line
Adjust the line using the handles if needed
Alternatively, type three hyphens and press Enter for a horizontal line
Alternatively, type three underscores and press Enter for a thicker horizontal line
Alternatively, type three asterisks and press Enter for a dotted line