When you feel good about the flow and are ready to start seeing your brand, colors, and look come together, it's time to starting building the aesthetic design that your users will fall in love with.
We always like to start broad with what we think our users like, what apps they use the most, and which ones they rave about that are in popular categories. We create a "moodboard" out of these styles, fonts, colors, and designs, and present them to the product owners for direction to the designers.
Most design is trending towards flat at the time of this writing, but there is a surprising amount of options within flat. Flat design doesn't necessarily mean no depth to a design. It utilizes spacing, imagery, font choices, and size, and sometimes a blurred background to achieve depth. Flat design tends to be extremely clean and pays great attention to negative space. This is much different than the skeuomorphic designs of a year or so ago. Designs were heavily textured, used a lot of shadows, and looked like what they represented in real life.
This is where designers will spend the most time, ensuring that everything is ready for prime time. If you are fortunate and have design and development under one roof like we do, then this is also the time when we begin to get started on the engineering deliverables. The engineer and designer work directly together during the production process.