This work was recently added to the Internet Archive site. I remember looking for a copy online last year, but it was unavailable for free. Thanks to Sean McDonald for bringing it to my attention!
Matthew McMahon has republished this work here if you would like a printed copy.
This is from the Lulu site concerning McMahon’s reprint: “Thomas Ford (1598–1674) was a Calvinistic, Reformed nonconformist divine, who sat on the Westminster Assembly. The Puritans believed in Exclusive Psalmody – and the Westminster Confession demonstrates their position. However, there are few works that were written as a whole explaining why this is so by a Westminster divine. This work by Thomas Ford does just that. His views demonstrate the majority view in Christendom up and until his era, and he sits in company with the best theologians and preachers through church history on the subject. He covers, 1. That we must sing. 2. What we must sing. 3. How we must sing. And, 4. Why we must sing. His main text is Ephesians 5:19, “Speaking to yourselves in psalms and hymns and spiritual songs…” This is an extremely valuable treatise on God’s form of instituted worship and the regulative principle. This is not a scan or facsimile, and has been updated in modern English for easy reading. It also has an active table of contents for electronic versions.”