Name the recurring pieces
Identify headings, navigation, buttons, forms and notices that serve the same purpose in different places. Define their behavior as well as their appearance. A shared color is not enough if the same control behaves differently on every page.
Give states a place in the system
Document focus, disabled, error and loading states with realistic content. Test components using a keyboard and at different text sizes. Include guidance about when a pattern should be used, not only how it looks.
Keep the system proportional
Start with the pieces the product already needs. Record deliberate exceptions and remove patterns that nobody uses. A small, maintained collection is easier to apply than a large catalogue copied from a theme demo.
FOLLOW YOUR CURIOSITY