Ask HN: How to learn on basic system design/modelling?

So I'm a recent IT grad, currently interested in being more proficient in designing systems and modelling systems. I don't really know where to start. Googling 'system design' or 'system architecture' returns results for scaling big systems.

I'm currently at the stage where I want to learn common/formal processes to translate business process into a system design/db models/app. For context, I would like to be a consultant to small-medium businesses to help automate their processes.

Some examples that I personally have an opportunity in are:

- Customized POS solution for restaurants (I have source code access to a currently running software, would like to reimplement to be web based)

- Hotel/lodging management solution for small establishments

- Custom systems for medical practices management (dental practices etc), handling customer information, bookings, and customer history

I'm aware that I'm probably not going to do all that in a short time, but there is still a huge gap in my mind between knowing the business process and actually implementing it correctly for the business. Any pointers or informations for things to read/watch are greatly appreciated.

Thank you!