Iam making an attempt to answer your query
TO MAKE DAHI:-
First boil milk adding half glass water for cleaning the milk packet. After the milk is boiled allow to cool till luke warm and then take two cups of packet curds or previous days curds or buttermilk , and add to the milk by stirring well. Keep it aside for 6- 7 hours. Your thick curd or dahi is ready. Preferably its better if you do it in the night before going to bed for best results of dahi
TO MAKE SWEET MANGO PICKLE:-
First wash the mangoes and dry it. then cut the mangoes into small pieces or any shape of your choice.
Take a bottle with tight lid . Now put one layer of mango pieces and one layer of salt, then again one layer of mango and another layer of salt, repeat the process for all mangoes and keep it aside for a week. In between slight shake the bottle to mix up. when the mango pieces change to slight yellow colour then make a fine powder of mustard seeds with redchillies. Now add the powder to the mango bottle, mix it well and keep aside for a week to absorb. If you want the pickle to be sweet then add sugar to the mixture and then keep it for a week or more to absorb and make the pickle delicious.
QUANTITY OF INGREDIENTS:
For big size 4 mangoes you can buy 100 gm mustard seed and 200 gm Red chilly [guntur is specially preferred for pickle]
I hope it must have helped you.