well black powder is just a mixture of charcoal (you need wood and mud to make a charcoal pit), saltpeter (aka potassium nitrate), and sulfur. nitrate is found in many household products - hence the bathtub chemistry. there’s a lot of that stuff all over the place, it’s in just about everything - from salts, makeup products, fertilizer, plant food, etc. I’d use that first while building a natural extraction process since it takes a long time, months usually.
you’d take dung from animals (horse works well or cow patties), wood ash, dry straw, and lots of urine (barrels of the stuff). a few months later you drain off the liquid, boil it with finely ground charcoal and then filter it with cloth. simmer the strained liquid until it’s reduced by 3/4, you want it really concentrated. spill it out onto very shallow pans and let it dry to crystalize the nitrates. sulfur is readily obtained from many sources - usually volcanic but these days its in a lot of building/construction materials like gypsum. basic chemistry will allow you to extract it. I live near geologically active areas, so sulfur would be relatively easy to obtain in the future.
there’s no real set recipe for black powder, but approx 75% dried & powdered nitrates, 15% dried and powdered charcoal, 10% dried and powdered sulfur. you want to grind each individually and then mix them together in those ratios. now you have black powder. keep it dry and away from spaks/flame/heat - bull horns work really well for this.
casting shot is just melting lead in a specific form - my father probable still has his old tools for that but if he doesnt they’re not hard to make. the only tricky bit in the whole process of building your own diy black powder rifle is the barrel - you have to hand forge those around a mandrel (a rod of a specific thickness) - I’d use high grade steel rods for that, since they’re just laying around. blacksmithing is usually a two or three person setup. once I had a water wheel constructed I’d probably build a hydraulic powered trip hammer to make things easier.