Mix 200 gr of the rye and 200 gr of water together with a spatula and leave outside of the fridge for a day.
Day 2 to 6
Each day add 50 gr of rye and 50 gr of water to the mix and again let it stand outside of the fridge. After day six keep it cool at around 10C. A cellar would be ideal but ofcourse a fridge would work too if you do not have a cellar.
You will start to see something happening after about 2-3 days.. The volume will expand and there might be airbubbles...