As a TA for CS195i, Designing, Developing and Evaluating User Interfaces , I put together a resource for students in the class on creating a design language. As many of the students in this class are extremely talented programmers, but have not had much exposure to the design process, I thought it would be valuable to present an approach to design that is more quantitative.
After walking through many of the common techniques and practices of UI/UX design, I present the concept of a design language as a document that communicates visually, and quantitatively your proposed design. The intention is that this document guides the design of new features to your product in the future, and allows you to work more closely with developers during implementation.
You can find the full document here.