How should you approach user interface design when you start a project?
Being aware of the difference between planning and creating user interface is the first and may be the most important step. The former requires the skills of a startup expert who has enough experience in the domain and knowledge of the target audience - often called a Product Manager. The latter is done by a skillful visual arts designer.
The output of planning is something that looks like the image above. These are simple sketches of your product’s screens, which include all the functionalities and all possible scenarios. They are called ‘mockups’.
The output of creating a UI is the actual high-res images that will be used by the developer to develop the product. The designer creates the UI based on the mockups. Keep in mind that these high-res images will be handed to a developer, so it is always safer for the designer and the developer to agree on the format before the designer starts working.