One easy way to do this is to put a border on the paragraph (either a top border or a bottom border will work, just be aware of which paragraph you put the border in):

1. Place the cursor in the paragraph.

2. Choose Borders and Shading from the Format menu.

3. On the Borders tab, select a line style.

4. Click the buttons in the Preview area (you can choose top, bottom, left, or right borders).

You can also draw a graphic line with the Line tool:

1. Choose Toolbars from the View menu.

2. Select Drawing to display the Drawing toolbar (it usually appears at the bottom of the screen).

Note: There is usually a Drawing button on the Standard toolbar that will display the Drawing toolbar.

3. Click on the Line tool to activate it.

4. Click and drag in your document to draw the line.

Hold the Shift key while you drag to draw a perfectly straight, horizontal line.

It's best to add graphic elements after you have completed the editing of your text, then you don't have to worry about them moving to undesired locations as you edit.