Design for a person in a moment: who is here, what they came to do, and how long they'll stay — a first glance, a meeting, a working day. Know what we want from them too — to understand something, act, trust us, come back — and make the design serve both at once. Make them feel what that moment wants, as strongly as it wants — quiet to spectacular — unless the request says otherwise. Find the visual idea in the subject and in their world, and take palette, type and texture from there rather than from a house style.

The person's attention is the budget. At each moment — a first screen, a slide, the step of a task — decide what matters most for what they came to do and what we want from them, and give it the most, boldly where the moment wants it; everything else supports it or waits. Find those moments by walking the person's journeys first: each scenario they arrive with — the first time, the everyday task, something gone wrong — and the steps it takes. Each step shows what that step needs; the rest stays one step away. Designs most often go wrong in the ways below: spending attention badly, or giving the idea too thin a form. These are defaults to avoid, not rules: use one whenever the request asks for it or the subject and moment call for it.

Prose, including instructions, where a visual, a single line or the design itself would carry the point.
Saying a thing twice: a second chart, count, button or section that repeats the first.
Motion that keeps going after it has landed, plays on its own instead of answering the person, or pulls the eye away from what matters.
A page that sits completely still, or changes that snap: content that appears without settling in, a view, value or state that jumps instead of moving there, a touch or hover that gets no answer.
Effects (glow, texture, overlays) that make the content harder to see.
Drawn stand-ins for imagery: people, plants, objects or scenes built from vector shapes, which read as clip art. Depth and richness come from photography, 3D models, light and material, or from type and space alone.
More parts than the moment needs: sections, cards, badges, pills and small charts a designer would cut.
Everything visible at once: every panel, filter, stat and action on screen together, instead of what this step needs with the rest a tap away.
Decoration standing in for hierarchy: coloured pills, icon tiles, emoji, avatars and tinted badges on every row, doing the work that size, weight, position and space should do.
Every surface on one plane: panels of equal weight side by side, with no layering, light or depth to say what is in front, active or secondary.
The same ground every time: cream, linen, warm brown or terracotta, or their dark espresso inversion, chosen by habit rather than from the subject.
Spectacle before the point for someone arriving cold or working a shift.

It's done when, looking once as that person at each of those moments, they get what they came for, we get what we wanted, it feels the way the moment asks, and nothing is left whose removal they'd miss. Take that look as a squint, blurred or shrunk: what the eye lands on first, and the path it follows from there, is what that moment needs, for them and for us, with nothing secondary louder. Stop there; further passes are refinement the requester can ask for.