Divide the milk into 2 medium saucepans and place them over medium heat. Allow the milk to get to the point where the top of it is covered in tiny little bubbles, and it looks like it might start bubbling over. How long this takes depends on your stove and the heat you've place the milk on - anywhere from 5-10 minutes.
Once the milk is gently bubbling, turn the heat off and add the lemon juice. Stir gently, only just to distribute the acid in the milk. Allow to sit for 15 minutes – the mixture should have curdled and become obvious ‘curds and whey.’ There should be ricotta like lumps of milk and a semi-transparent lemon coloured liquid.
If the liquid is still milky and completely opaque, return the ricotta to a gentle heat until it begins to separate, and then take it off the heat again. You can add a little extra acid (a teaspoon at a time) as an insurance policy, although this might affect the taste of the final product.
Once you have left the mixture to sit for 15 minutes, the mixture should have obviously separated into the two distinct elements. Gently pour the ricotta through a sieve lined with muslin cloth or a nut milk bag. You can discard the whey or use it in smoothies or baking – see notes for a few more ideas.
Strain the ricotta for around 5 minutes - just enough to get a bit of liquid off. This quantity should make around 400g+ ricotta. Once it’s drained, place it in a large bowl.