Depends how sour you want it. I would probably replace some amount of water with apple juice, maybe half the water with apple juice, which I would guess would give you an OG around 1.030-ish, then use malt to bump up the gravity to where to you want it. I'd probably mash pretty hot, maybe 158*.
After the mash, I'd inoculate the wort with Lacto, wait a few days, (or just one if you just want it tart), boil the wort, then add the juice and the yeast once it cooled.