Writing alt text for people, not checklists
The usual advice on alt text is to describe the image. That is a start, but it produces text like "A woman smiling at a laptop in an office", repeated on thousands of websites. A screen reader user hears this and learns nothing.
Ask what the image is for
Before we write anything, we ask what the image does on this page. If it is decoration, an empty alt attribute is the right choice, and the screen reader skips it. If it carries information, such as a chart, we say what the chart shows. If it is a product photo, we say what makes this product different from the one on the next page.
Three examples
A photograph of a guesthouse room. Not "room with bed", but "Double room with a wooden bed, a window facing the forest and a woollen blanket on a chair".
A graph of monthly sales. Not "graph", but "Monthly sales rose from 120 to 340 units between January and June, with a dip in April". The same text goes under the graph, for everyone.
A logo that links to the home page. Not "logo", but "Tammik Studio, home".
Check it by listening
Every few months we turn on a screen reader and use our own site with the screen turned off. It takes ten minutes, it is sometimes humbling, and it finds problems that no automated tool will.