Design systems sound like something only big companies need. In practice, small teams benefit the most, because they cannot afford to redesign the same button five times.
Start with tokens
Colors, type sizes, spacing and corner radius. Give each one a clear name and use only those values. This alone makes a product feel consistent.
Then the ten parts you use everywhere
Buttons, inputs, cards, tags, navigation, modals. Build these properly once, with all their states.
Keep it in one place
One Figma file and one code library, kept in sync. When the system and the product drift apart, the system stops being useful.
Grow it slowly
Add a new component only when you have needed it twice. The best systems are small and actually used.