Before diving into the visual principles of color, typography, and layout, itís important to talk about basic usability. The most aesthetically pleasing design in the world is useless if itís not usable.
One of the most important usability principles is the idea of consistency or predictability. Designs should be predictable enough that users are able to intuitively understand how to use them. For example, blue underlined text for clickable links, navigation menus that are complete and well-labeled, etc. Spacing between elements, typography, and color scheme should also be consistent.
Other usability principles that should be considered in every design project include things like error prevention (and informative error messages when errors do occur), familiar language (use the language people are used to, rather than ďcuteĒ or creative alternatives that might be unclear), flexibility and efficiency, and easily available help. Nielsen Norman Group has additional usability heuristics that should also be kept in mind.
Usability evaluations should be conducted throughout the design and development process to make sure the product functions the way the design and development team intended, and that users are not confused. Heuristic evaluations involve comparing a list of predefined design principles that a product should be following with the actual product to see where deviations occur (and then fixing those deviations).
Once usability has been thoroughly understood in relation to the product at hand, designer-developers can move on to the more visual aspects of the design.