Many (graphic) designers do not understand the needs of a user-interface, one needs to do quite a lot of research and ask people to try out things - 'hands off' - and see what they do, what confuses them, what mistakes they make.
Most of the advice gives three steps to user-interface design:
content or wireframe - what is in the interface,
flow or relation - how the what links, and
style - how it looks.
The topic is huge, there are good links previously posted, Coopers book 'About Face' although a bit wordy has explanations of various gotchas.
