It wasn't line-less paper.

Timothy Matlack was the gentleman tasked with drafting the original Declaration. He likely used ruled lines that are simply no longer visible due to handling and the passage of time.

Pin pricks and line rules were often used during that time to guide a scribe's hand. The document would have been plotted out before actually written on: margins (the ruled lines which which you can on the Declaration, in fact still see), font size and ruled lines to keep the text straight. He did not write it free handed.

Handwriting in America: A Cultural History By Tamara Plakins Thornton

https://www.archives.gov/publications/prologue/2016/fall/declaration