Just inputs, also known as just frames and all those things you said above, work mostly the same way as normal moves. The difference is that they requires a very precise timing in order for the move to come out as advertised. Most of these also make your character glow white upon successful execution so that you know you did it right.
The game runs at 60 frames per second, and most just inputs have a 3-frame window or less for successful input. This means that you have one twentieth of a second or less to put this input in. The exact window is dependent on the specific move in question.
Mashing the button as fast as you can is not a viable strategy for just input. This is a finesse mechanic, and not a brute force mechanic, and while I'm not 100% sure of how the engine handles mashing just inputs to keep them ineffective, I am 100% sure that it is not a reliable way to get these to work.
As for strategies on how to learn these inputs, keep trying different timings until you get it right. Once you do, you have to learn the rhythm of the move. Most just inputs are followups to other moves you can use, and thus inherently give you a basis to time your attack off of. Each just input has a different rhythm, but if you practice one enough you can get surprisingly reliable at it.