A design system sounds like something for companies with a dedicated team and a large budget. In practice, a small system pays back fastest for small teams, because every decision made once is a decision nobody makes again.

Start with the parts you repeat the most: colour, type, spacing, buttons, forms and cards. Write down the rules in plain language next to the components so that a new designer or developer can use them on day one.

Resist the urge to build for cases you do not have yet. Add a component the second time you need it, not the first.