Even if you have a clear idea of what you are designing, it is important to research this further. For example, if you are designing a website for a cleaning company, it is important to know exactly the kind of cleaning work it does. It is also important to analyze the competition to ensure what you design is in keeping with the general standard for your sector, and, when possible, exceeds expectations.
You are producing the design work for your clients (or for you). It is essential to discuss with them from the beginning what their expectations are, so you can satisfy them. Bear in mind that you are also designing for the consumer -- the person who will purchase and use the product. What are the customers' expectations? Market research and focus groups can help determine what consumers hope to get from a product or service.
You must be clear as to the outcome of your design. If the design is a website offering a service, the desired outcome is that consumers will browse and purchase from the website. If you are designing a child's toy, you must ensure it is suitable, entertaining and meets health and safety regulations for the predetermined age range.
Consider the usability of your product -- that is, whether it is efficient and uncomplicated. Does it provide good value for consumers seeking bargains, or are you aiming for the higher end of the market? Will it sell well relative to other businesses competing in the same industry?
The biggest advantage of documenting your design objectives throughout the process is that it will significantly cut down the need for changes. However, from the initial design to the fully developed product, some changes likely will necessary. Your client may require changes, and, as you proceed, you may discover ways to make the product work or look better. Changes should be documented, tested and signed off both by you and by the client.