Pour the milk into a large stock pot or very large saucepan and place over medium-high heat until it is just below the boil (should be around 100C/200F if you have a candy thermometer you can use to test it - or just look for the first few bubbles to appear). Take off the heat and pour in the citrus juice. Stir well until the milk separates into curds and whey. If it doesn't all separate, place back over the heat for a little longer and add a bit more lemon juice and stir again until the curds are all separated and floating on the surface.

Pour the contents of the pan to a muslin-lined colander set over a large bowl and allow to strain for 15-20 minutes, until the whey has strained into the bowl and left just the curds in the muslin. The longer you leave it, the drier the ricotta will be, so if you are like me and like your ricotta creamy, check on it after 10-15 mins and move it to an airtight container in the fridge as soon as it is the consistency you like! If it's gone too far and looks drier than you like, mix a little of the whey back into it to loosen it. (And while you are at it, DO NOT CHUCK THE WHEY! You can do so much with it - see this post for 36 suggestions!)