Add 1 litre unsweetened soy milk to a large, heavy-bottomed saucepan over medium heat. Cook for around 10 minutes, stirring occasionally, until it comes to a semi-simmer and a slight skin forms over the milk
Add 1 ½ tablespoons apple cider vinegar to the pan. Begin to stir in one direction, and cook for a further 2-3 minutes, until you begin to see whey* (green liquid) separating from the soy milk curds. Turn off the heat and leave it to rest for five minutes.
Drape a cheesecloth over a sieve. Pour the contents of the pan into the cheesecloth, which allows the whey to drain through while the curds stay in the cheesecloth. Reserve the whey. Run cold water over the curds to wash off any excess vinegar taste, then gather the top of the cheesecloth and squeeze to remove excess water from the curds.
Transfer the tofu curds to a tofu press. Leave the cheesecloth wrapped around the tofu when pressing. Close the press and leave for approximately four hours (for extra-firm tofu).
Notes
* If the curds don't separate from the whey, add more vinegar incrementally. If you don't have a tofu press, you can create a make-shift one with a large plate to catch excess water, and heavy weights (such as cans) on top. This may take longer than a specialist tofu press, which is a great investment.