Place the cursor where you want the line
Type three hyphens and press Enter to insert a horizontal line
Type three underscores and press Enter to insert a thicker horizontal line
Type three equal signs and press Enter to insert a double line
Go to the Home tab
Select the Borders drop-down in the Paragraph group
Choose Horizontal Line
Go to the Insert tab
Select Shapes
Choose a line shape
Click and drag to draw the line
Go to the Insert tab
Select Table
Insert a one-cell table and adjust the border to create a line
Go to the Design tab
Select Page Borders
Add a border line to the page
Use the Draw tool if available to draw a custom line