When I practise, I usually practise elements, like the ascender loop, or the entry stroke into the s, or the downstroke of the t, or the corner curve into its exit stroke. To me, writing Spencerian is mostly about chaining (and executing) these elements the right way. I do practice complete letters and words from time to time (tiptop, soliloquy, assistant, black), but it's not what I tend to focus on.
Does it work? I don't know. Frankly, I have no idea what my script looks like. Possibly it could use following an exemplar a bit closer, and I have an enormous respect for people who are disciplined enough to do that.
On guide sheets, I think using guide sheets for a while can be beneficial, but there are also techniques to build slant consistency without them that might be worth trying out.
If I'm not mistaken, Madarasz retouched too.