Freelance UI designer new to coding. Where do I begin?

Hello there, I'm a UI designer who is freelancing. I've been passing Figma files on to developers for 6 years now, and getting nothing but the runaround. So I decided to start creating my own little tools recently.

The first one I created was an invoice generator using Next.js. It is ugly, and every time I launch the terminal, it feels as if I'm wearing somebody else's shoes.

What would be the advice you would give to those who used to be just designers and switched to dev? What helped you – building small things or structured education? And what mistake would you advise your past self not to make?