How do you practice system design alone? Videos aren't doing it for me

I've watched a lot of design walkthroughs. I can follow the diagram, but when I start from a blank page I just draw boxes and stop.

Right now I pick one app I use daily, write requirements for ten minutes, then estimate traffic on paper before I touch any boxes. Is there a better routine? I'd especially like to hear from people without a senior engineer around to critique them.