A classy skirt outfit is really just about finding the pieces that do the quiet, grown up version of dressing well, something you can wear to a lunch, a christening, a work thing with wine involved, without it ever looking like you tried too hard or not hard enough. The whole point of a good skirt is that it gives you a shape to build around, rather than something you have to fight into shape yourself.
What it pairs with matters more than the skirt itself, honestly. A fitted jumper or a simple blouse tucked in gives you a clean line that works whether the skirt is straight, pleated or has a bit of swing to it, and a blazer over the top turns the whole thing into something you could wear to nearly anything without changing a stitch. I've found the midi lengths are the most forgiving for layering, because they leave room for a chunky knit in winter and a thin camisole in summer without the proportions going wrong. Shoes do a lot of the talking too, a low heel or a neat flat reads considered, while anything too trainer like can undo the effort of the rest of the outfit.
On colour, the ones that are easiest to live with are the ones that don't ask much of you in the morning, so navy, charcoal, a soft camel, black that isn't too stiff or shiny. They go with nearly everything already in your cupboard and they don't date the way a very trend led print does. For fit, the thing to watch is the waistband, because a skirt that's tight there will nag at you all day in a way that's hard to ignore, and one that's loose will twist around so the seam ends up at the front by lunchtime. Check how the fabric sits when you sit down too, some skirts ride up, some pull at the hips, and you only find that out properly once you've been sat at a table for twenty minutes, not standing in front of a mirror.
If you're starting with nothing in this category, a straight or slightly A line midi in a fabric with a bit of weight to it, not flimsy, not stiff, is the one to get first, because it holds its shape through a whole day without creasing into something sad by the evening.
Keep anything with drape or pleats away from the tumble dryer and let it hang dry instead.
