Open your Google Docs document
Place the cursor where you want the line
Go to Insert
Select Horizontal line
Use the toolbar or keyboard shortcuts to add a line if needed
To draw a line, go to Insert
Select Drawing
Click New
Choose the Line tool
Draw the line
Click Save and Close
To add a line using borders, select the paragraph
Go to Format
Select Paragraph styles
Choose Borders and shading
Apply the line style you want