It usually takes a few years before the release of the first installment of FD. I usually replay the first installment and then play FD again. If I play FD directly, I will forget the plot of the first installment and feel disconnected.
Just play fd is my choice. And if fd was good, I may replay original game as well. I rarely read games the second they are out, so when the time for reading comes, not only fd is published, but sometimes even sequel is already available.
If a fandisc comes out years after the main game, I've probably already forgotten most of the story by then. I'll go look up some game reviews or check the CGs I saved to help refresh my memory. I don't usually replay a whole game.
I'm usually a latecomer, so I tend to play both original and sequel/fandiscs back-to-back!
If I have left some note/screenshots previously, I'll reread those; if I remember absolutely nothing, I might skim through reviews instead! Though I rarely end up needing to.