A simple guide:
More detail. More drama!
The simplest flash possible
The simplest, smallest, and most automatic flash, the better.
For example, RICOH GR II in program mode with the integrated flash.
Experiment with distances
Photos from my SUITS project. Some shot on film Leica + Kodak Portra 400 flash + Leica SF20 flash. Also a few on RICOH GR II on flash.
Practice with in-animate objects and people.
Experimentation is key
You never know what a flash street photo will look like until you shoot it. Thus, just shoot in order to see how the photograph will look like with a flash.
Experiment with different cameras and formats
For example experiment with your iPhone, integrated flash on your Fuji or RICOH GR II camera, etc.
Monochrome vs color?
Free inspiration for you: