A place where reality is already weak. Other places are inflicted like wounds. A magical catastrophe, a mass slaughter or the worm-trail of a living dungeon could all weaken reality enough for the purposes of a hellhole.
You need to conduct an arcane ritual. A very big arcane ritual, involving lots of sacrifices and magical power. It’s the same sort of spell you use to summon a Demon, but instead of calling up a named demon, you’re opening the way for all of them. Alternatively, if there’s enough magical energy sloshing around nearby, it may pool in the weak spot in reality and erode the fabric of existence until it frays through into a hellhole.
stand well back, because if your ritual works, you don’t want to be standing there when the hellhole opens. At the dead center of every hellhole (well . . . . most hellholes) is a gate to the hells or to the Abyss. There aren’t any binding circles or containment pentagrams around that gate, so the chaotic energies of the demon realm leech through into our reality, warping space and time. As long as that gate stays open, the hellhole exists in all its impossibility; the bigger the gate grows, the bigger the hellhole.