Start with a general statement of what the image shows and then go into more detail about specific elements. For example, if the image shows a deskphone, the first statement in the alt text should indicate the deskphone model and then you can go on to describe the layout of the phone.

When writing alt text, try to describe the image elements in the order that is most useful to a user. For example, describe the most frequently-used elements first. When in doubt, describe the elements from top to bottom, left to right, or however the elements are most logically oriented in the image.