Here is how to make tangzhong. This recipe gives 200g of tangzhong.
50g (⅓ cup) bread flour
250ml (1 cup) water
Mix flour in water and stir well so you don't have any lump. Cook on medium-low heat, stir consistently with a spoon or whisk to prevent the mixture from burning and sticking while you cook it.
The mixture will become thicker and thicker. Once you notice lines start to appear in the mixture for every stir you make with the spoon the tangzhong is ready or if you have a thermometer use it to check the temperature. Once it's 65degrees it's done. Remove it from heat.
Pour this mixture into a bowl and cover with a cling film sticking onto the tangzhong. Leave to cool to room temperature.
This can be used as soon as it cools or stored in the fridge and used within two days. Always bring back to room temperature before use.