... there are quite a few advanced Spencerian writers here.
... and then there's me, a complete duffer.
Nevertheless, I've had the privilege of meeting and getting advice from a world class penman, so I'll pass on the essence of his approach. Find an exemplar: we used
Spencer's New Standard Writing - its Principles, proportions, classification and analysis (1887) but
Real Pen Work is another good one. Ignoring the capitals at first, study the letterforms in the minutest detail, and then start to reproduce them individually being intensely self-critical. Be very aware of the main and connective slants, the shapes of lead-in and exit strokes, and proportions in general. Do not content yourself with anything short of perfection! Don't bother with shading except for t, d and p. Cultivate muscular movement - the fingers should be immobile for everything at this stage (except loops - arguably). Push-pull drills are good for this. To develop arm movement for capitals, work large (2" high), use a pencil or felt tipped pen at first, and do oval drills on the main slant and horizontally.
The most important things are observation and ferocious self-critique. It's a tough regime, but it does work to the extent that there was a less than subtle improvement in my efforts in the space of a six hour seminar, and it continues to pay dividends. What I'd add is that the above applies to practice: you can lighten up when you write a letter or whatever, otherwise you'll drive yourself mad.