Take a large pitcher or bowl and squeeze fresh lemon juice into it. I like using a lemon squeezer to get the maximum juice out of the lemons. Use juice of 2 lemons here, if they are super big then you can use 1 or 1 & 1/2 as well.
Add in 2-3 tablespoon sugar (to taste), 1/4 teaspoon black salt (kala namak), 1/8 teaspoon each of salt and chaat masala. Add in 500 ml chilled water.
Using a wire whisk, stir for few minutes until the sugar dissolves and everything is well combined.
Serve nimbu pani with fresh mint leaves and lemon wedge. You can also add ice cubes though I usually don't since that dilutes the flavor of the drink which I do not prefer.
Notes
You can use any sweetener of choice here or you can even skip the sugar. However, nimbu pani is supposed to have a little bit of both salt and sugar.
You can also add roasted cumin powder here.
You can make this in advance and keep it refrigerated.
As mentioned above, you can add sparkling water/soda in place of plain water.
Lemon or lime both work really well, just make sure to use fresh juice. Bottled lemon juice doesn't taste as good as fresh.