*In general* against loose passive players, when playing low stakes no limit and you flop the nut flush and get bet into on the flop. Is it best to smooth call or raise?

I would think it's best to fast play incase the turn brings another flush suit and kills the action.

Lately I've been 2.5 x'ing leads into me, and villians have just been folding

This is good problem to have.

Can you teach me how to flop the nut flush often enough to have to ask this question?

Flat and let them bet into us again, and raise the turn.

We want to maximize value from our one-in-a-hundred miracle flops.

I hate to give the “it depends” answer but it really does. Deeper stacks make me more likely to raise. My image being aggressive makes me more likely to raise. Since you described them as loose and passive that makes me want to raise more since we don’t have to worry about raising them off their bluffs since they are value heavy. Board texture also matters, are they likely to have a set or two pair, or other strong value hands they will continue with?

If I literally just sat down and have no info I generally flat since I think people play pretty nitty on monotone boards but it really depends.

This. /thread