It’s just Bayes, but I’ll give it a shot.
You’re having a conversation with someone. They believe certain things are more probable than other things. They mention a reference class: if you look at this grouping of claims, most of them are wrong. Then you consider the set of hypotheses: under each of them, how plausible is it given the noted tendency for this grouping of claims to be wrong? Some of them pass easily, eg. the hypothesis that this is just another such claim. Some of them less easily; they are either a modal part of this group and uncommon on base rate, or else nonmodal or not part of the group at all. You continue, with maybe a different reference class, or an observation about the scenario.
Hopefully this illustrates the point. Reference classes are just evidence about the world. There’s no special operation needed for them.
Yes, your understanding matches what I was trying to convey. The feedback is appreciated also.