I buy readymade puris. You can make them as well at home though that is time consuming.
Keep all the veggies chopped and make the chutneys. You can also buy date and tamarind chutneys in the market if you don't wish to make them.
Beat the curd and add black salt and sugar to it. Taste and add more salt if needed. It should be very slightly sweet.
To assemble:
Make holes in the puris.
Fill a little potato onion mixture in it.
Now put in about a tablespoon of beaten curd. Add the sweet and green chutney.
Sprinkle some chaat masala.
Now sprinkle some sev on top and serve immediately.
Your dahi puri is ready to serve.
Notes
As soon as you assemble it, have it. You can't store it as the puris will get soggy.Adjust the chutneys as per your taste. You can do away with the sweet chutney if you don't like it. You can also add boiled sprouted black gram or moong sprouts to the filling if you like.