Fastest way to get clarity on a project is putting pen to paper. Saves time before going to higher fidelity tools like Illustrator, Photoshop, Sketch, Azure etc.
User Flow Diagram
Using illustrator or OminGraffle to workout user flow and structure.
Ui design can begin in earnest with wireframes based on feed back garnered in previous brainstorming sessions. These can be as high fidelity images as needed. Easy to see the flow and see how the pages will look static. Style choices can be made here.
Back to the board
Sometimes it’s better to get things up on a wall where they can be quickly arranged and moved and get stake holders input in real time. Good way to step back and see the forest.
Higher fidelity Mockups
Higher fidelity UI design can now move forward to develop a working prototype. At this point a clickable prototype may be built for user testing.
Once a prototype is developed (or even a series of images or wireframes etc), user studies are a crucial part of development. Does the product or App work the way you intended? Sticking points? Confusion? It all gets sorted out at this stage which will save a lot heartache down the road into production.
Final UI design Validation
Final working prototype where you see results of all the previous input and user study work is utilized and make adjustments as needed.