We create a conceptual design, user testing each iteration. Such a flexible approach is handy when requirements change fast and there is no detailed specification.
To discuss the issue
more about Iterative prototyping
An iterative prototype is fast to create and fast to user test. It helps to check the conceptual structure, avoid excessive drilling down and save time for more significant modifications.
We change the interface concept, fleshing it out with details from one iteration to the next (1-2 days per iteration). We build user goals solutions into the interface architecture, discarding all minor issues. Preparing the prototype for user testing.