Not the way my MIL used to make uppumanga. Just was the green mangoes thoroughly, layer them with rock Salt, turmeric powder and green peppercorns. Seal the bharani well and don’t open for 2 weeks. After that open it give it a thorough shake and seal it again for 2 weeks. Enjoy with kanji or rice.
It would be appreciated if the proportion of salt and water is specifically mentioned i.e. how many Litres of water for how many grams of salt. Quantity should be EXACT. Not APPROXIMATE. You probably need to purchase a weighing scale and a measuring jar for this purpose if you don't have them. Or else the quantity of salt maybe more than required and the mangoes may turn bitter as you suggested.