Partially-drying the grapes first made them sweeter.
Some sources say Passum could also be made from must, but that would make in effect make it defritum or sapa, or that it could be made from wine, but that would make it Caroenum.
Boil grape juice, white or red, until reduced by half. Sweeten with sugar.
Literature & Lore
“Passum optimum sic fieri Mago praecipit, ut et ipse feci: uvam praecoquem bene maturam legere; acina mucida aut vitiosa reicere; furcas vel palos, qui cannas sustineant, inter quaternos pedes figere et perticis iugare; tum insuper cannas ponere et in sole pandere uvas et noctibus tegere, ne inrorentur; cum deinde exaruerint, acina decerpere et in dolium aut in seriam coicere; eodem mustum quam optimum, sicut grana summersa sint, adicere; ubi conbiberit uva[s] seque impleverit, sexto die in fiscellam conferre et prelo premere passumque tollere;  postea vinaceos calcare, adiecto recentissimo musto, quod ex aliis uvis factum fuerit, quas per triduum insolaveris; tum permiscere, et subactam brisam prelo subicere; passumque secundarium statim vasis oblitis includere, ne fiat austerius; deinde post XX dies, cum deferbuerit, in alia vasa deliquare, et confestim opercula gypsare et pelliculare.” — Columella, De Re Rustica. Chapter 12, 39.
“Mago advises that the best passum is made in the following way, and I myself have made it like this. Collect early grapes when they are fully ripened, discarding mouldy or damaged ones. Put stakes in the ground 4 feet apart and join them with poles. Place reeds on top, and spread your grapes out on them in the sun, covering them at night so that they don’t get damp from the dew. When they are dried, pluck the stalks off them, and put them in a cask or large earthen-ware jar, cover them with the best must you have, so that they are completely submerged. When the dried grapes have taken in all the liquid that they will, [let sit] for 6 days then put them in a basket and with a wine press extact the passum. Afterwards stomp the leftover grape skins, adding must freshly made from other grapes dried in the sun for 3 days. Mix well (with the first passum extraction), then put this mixture through the wine press. Put the results of this second pressing in sealed vessels, so that it doesn’t become bitter. After 20 days, when it stops fermenting, decant into other vessels, and immediately seal the openings with gypsum and skins.”