Pour the milk into a heavy-bottomed pot and heat it over medium flame. Stir occasionally so the milk does not stick to the bottom. Bring it to a full boil.
2 liters milk
Curdle the milk:
Turn off the heat. Add the lemon juice or vinegar one tablespoon at a time, stirring gently. Stop adding acid once the milk splits and the white curds separate clearly from the greenish whey.
2 tablespoons lemon juice
Strain the curds:
Line a colander with cheesecloth and carefully pour the curdled milk through it. Collect the whey in a bowl if you plan to reuse it.
Rinse the curds:
Run cold water over the curds in the cloth for about 30 seconds. This removes any acidic taste and helps keep the paneer soft.
1 cup cold water
Press the paneer:
Gather the cloth, squeeze out excess liquid, and shape the curds into a flat disc. Place a heavy weight on top, such as a cast-iron skillet.
Set and use:
Press for 20 minutes for soft crumbles or 40 minutes for firm blocks. Remove from the cloth and use immediately, or store for later.