They take certain results as fixed, and then work with the historical materials to show that those results are compatible with originalism as they describe it. The key move here is to chose or devise a version of originalism with which the fixed points are compatible. Of course, when you do that you’re likely to run into intra-originalist controversies, as Ed Whelan’s responses to Calabresi indicate. But, structurally, these are works in the “no pain, no gain” mold, and none the worse for being that (as such — you might not like the general theory they offer, but you shouldn’t object to the work on “no pain, no gain” grounds).
Mark Tushnet opines on constitutional theory.