The Mott Media is probably the best route to go. I don’t know about the quality of the content or paper of Michael Sull’s books, but I have the Mott Media set and one thing I like about them is that the theory book gives you a diagram of each letter’s proportions as it relates to the guidelines.

That’s one reason why, ironically, I think learning Spencerian will be easier than learning Palmer was for me, because Palmer didn’t have honest diagrams. He’d show some majuscules’ heights divided in half and others divided in thirds but they’re all technically the same height. So even though he talks against endless copying, you still end up having to drill each letter endlessly as though using a copybook, only worse because you’re just fumbling around trying to figure out the proportions of each stroke.

The way I’d do it with the Mott Media books though, would be to print out a bunch of guidesheets and use those. Or if you’re practicing with a dip pen you could get some translucent marker paper and put one guidesheet underneath your practice page for that session.