A "flow" is simply using the aeroplane as the the checklist: effectively in a "challenge / respond" manner rather than a printed checklist.
Most of us do our externals in that way I'd venture, and many instructors: myself included: teach that approach to emergencies.
Incidentally
Normal checklist one side, speeds and emergency stuff on the back. What more do you need?!
Most experienced pilots have their personal systems of-course. Mine is two 2-sided A5 cards, one generic, one specific to type, working like this:-
P1 - speeds / startup / limits / emergencies / key performance numbers.
P2 - systems knowledge, which pretty much never gets referred to in flight.
P3 - checks on the ground, I use this fairly extensively.
P4 - checks in the air, memorised, just there "in case".
So, in practice, P1 is the only page I look at with air under the tyres. In my experience, most experienced pilots of single pilot aeroplanes do something pretty similar.
G