In the evening, grind the seeds: Mustard seeds + Fennel Seeds + Black Salt + Fenugreek seeds together. Grind coarse, not fine. Let some texture remain.
In a big bowl/dish, add the dried mango pieces, then add the seed mix, add Kashmiri red chili powder, and 1/2 liter mustard oil. Mix well with your hands. Make sure your hands are clean and dry or any moisture in your hands will spoil the pickle. Pour some mustard oil at the bottom of the Jars and then pour this pickle mix into different Glass bottles/ Martabans / Bonechina Cylindrical jars. Add some mustard oil on top too. The Pickle needs to be dipped in enough oil as it acts as a preservative too. If your jars are airtight you need not add a muslin cloth on top otherwise to avoid dust going in, tie a muslin cloth on the lid.