After finding little information on inserting and adjusting horizontal lines in LibreOffice Writer, I came up with the idea of how to design my own custom horizontal line paragraph style and wanted to share it for others who might be interested.
Right click on the Default Paragraph Style in the Styles deck of the Sidebar and select New…. Give your style a name such as My Horizontal Line and select Default Paragraph Style or another paragraph style in the Next style section.
Next, go to the Borders tab and design a bottom border setting the Style, Color, and Width.
If you do not want the horizontal line to extend the whole width of the paragraph area of the document, you can go to the Indents & Spacing tab. Adjust the Before text and After text settings as desired. You can also adjust the Above paragraph and Below paragraph settings if you want extra space above or below the horizontal line.
Once you create this custom horizontal line paragraph style you need only hit the Enter key after a paragraph in your document, double click on your horizontal line paragraph style in the Sidebar, and hit Enter again to start typing a new paragraph beneath the horizontal line you inserted.
Let me know what you think of this method.