What I check before designing for a small screen
Process
A phone screen is not a smaller desktop. A short checklist keeps the first version honest.
I start every phone project by holding the phone. Not a mock-up on a large monitor, but the device itself, at arm's length, in the light I expect people to use it in. Most of my early mistakes disappear after five minutes of that.
Check the thumb before the layout
The first question is where a thumb naturally lands. Primary actions belong in that zone, and anything a person must reach for should earn the stretch. I mark the reach area on a printed screen and move buttons until the tap order matches the order of the task.
Text is the second check. I read a full paragraph at the smallest size the design will use, outdoors if I can. If I squint, the size is wrong, however tidy it looks in the file. Contrast gets the same treatment: a pale grey that feels refined at a desk can vanish on a bus.
Remove one thing from every screen
A small screen has room for one idea at a time. For each view I ask what a person came to do, then remove the element that competes with it. Often it is a decorative detail, sometimes a second button that says the same thing as the first.
When the design works on the phone, the larger layouts become easier. They add space and secondary content around a core that already makes sense, instead of squeezing a wide page into a narrow one.