My thoughts, in brief:
All you really need to learn from is an exemplar, and you'll find one in any of the Spencers' publications. They do vary, so best to have a look at a few. Plate 2 ("Medium Hand", on page 74 of my pdf file) in the New Compendium would do nicely. Thinking in terms of the seven principles does help - it's not essential but they can be used as building blocks for most of the letters and ought to make learning quicker.
Regarding guidelines, Spencerian is a practical hand and eventually you ought to be aiming to work from just a baseline. What you decide to use in the first place is up to you, but bear in mind that the writing does tend to spread out horizontally, and for that reason it's best kept quite small. An x-height of 3mm is large, but a good size to begin with ... 2mm is useful for everyday use.
Following from that, small writing needs a sharp pen. My preference is for very fine and very flexible but for modestly shaded textbook Spencerian you don't really need much elasticity, so it's a matter of personal taste. Nice thin ink is a must: iron gall, walnut, Chinese stick ink ... anything but acrylic!
I do not believe in a "proper" hand position, although there are plenty of improper ones. If you can produce something resembling engrosser's script I'd humbly suggest that you don't worry about it. If you insist on worrying, there are plenty of threads here about correct grips - and they're amongst the most argumentative and heated you'll find on this forum except for the ones about modern art. Please excuse me if I don't go there again.
Finally, general advice. If you're using the New Compendium, your aim is to write like Lyman Spencer. This is no easy matter, but if he could do it, so can you. Therefore, study the exemplars minutely; attempt to reproduce them exactly, and be intensely self-critical. That may sound harsh, but it's actually the easiest way.
