Building a design system can be helpful for enterprise teams who need to keep their teams working together within an ecosystem. A design system is not just a style guide or a grouping of reusable html and css. A design system is a system that informs the method in which a team builds systems and includes the documentation of the overall methodology along with processes, workflows and code for the design and development within the organization.
Where to begin
In order to document the design and development needs of an organization there needs to be some preliminary research to find out what the current state of design is within the current applications. In order to figure this out a design audit needs to be done to catalog the way color, buttons, typography, spacing, imagery, etc. are currently used.
An established and shared set of criteria on how to approach design across the organization should be created so the team understands how to best solve problems together.
A comprehensive set of usage guidelines regarding the brand identity (i.e. color, typography, spacing, grid/layout, icons, illustration, motion, etc.)
A glossary of commonly used interface elements and patterns that can be scaled to multiple products and use cases.
Documentation is a big part of insuring the system is easy to use. You should consider both the current team and team members that will be added in the future. The documentations primary goal should be to educate your team of how they should be using the system.