Peel the mangoes and remove the pulp.
Slice the pulp in any shape you like.
Transfer the slices into a large glass bowl and sprinkle salt on top of them. Set aside for 24 hours.
After 24 hours, strain the water and spread the mango pieces on a fine muslin cloth. Do not throw away the water. It is important to set aside this water.
Place the mango pieces with the muslin cloth in the strong sun to dry for 7-8 hours.
When the mango slices are slightly dry (known as फरहरा in Hindi) move them inside from the sun. Do not dry the mangoes completely in sun. It is important that they should be only partially dry.
In a large wok, bring mustard oil to a smoking-hot temperature.
When the oil is boiling, add salt to split the oil (known as तेल को फाड़ना in Hindi) and turn off the gas.
Allow the oil to cool down completely.
Once the oil is completely cooled, add mango slices, strained salt water, and pickle masala. Mix vigorously.
Transfer to a large glass jar or मर्तबान/ बरनी with an air-tight seal. Close the seal.
Place the ready pickle in strong sunlight for 15-20 days. You can place the jar every day at the time of strong sun and remove them indoors at night time.
After 15-20 days, aam ka achar (mango pickle) is ready to eat. Enjoy with mathris, poori or just like that.