Preheat the oven at 50º C or 120º F.

Pour the soy milk in a saucepan, add the agar agar, stir and heat the mixture to 90º C or 195º F, but be careful because it shouldn't boil. You can use a thermometer, but it's not necessary.

Pour the mixture in a bowl and let it cool to 40 or 50ºC (105º F or 120º F). If you don't have a thermometer you can put your finger in the mixture and when it's hot but not burn it's ready.

Add the starter (soy yogurt or yogurt starter powder), stir and pour the mixture into glass jars or containers.

Turn off the oven, place the glass jars or containers (without lids) inside and let stand for at least 8 hours. You mustn't open the oven during this process.

Now you can cover the jars or containers and keep the yogurt in the fridge.