Preheat oven to 50°C/120°F. Add soy milk to a pot and bring to a simmer. Transfer soy milk to a liquid measuring cup and let cool down to 44°C/110°F.
Add soy yogurt and stir to combine, then transfer the mixture to a glass jar and seal tightly.
Transfer to the preheated oven at 50°C/120°F for approx. 6 hrs. Afterwards, let cool down to room temperature and store in the fridge up to a week. Enjoy!