It's very easy to make tangzhong and it only takes a few minutes. Just remember to refrigerate it overnight or at least for 12 hours. It lasts for a couple of days, around two I believe. Don't use it if it turns gray.
Tangzhong helps make bread really soft without any preservatives.
To make it you need 250ml of water or, as noted by Christine on her blog, 1/2 water and 1/2 milk, or all milk and 50 grams of bread flour.
Dissolve the 50 grams of bread flour in the liquid.
Get rid of the lumps so that it looks like this.
Cook over medium fire until lines form.
Pour tangzhong into bowl and cover so that no skin will form.
Let it cool to room temperature and then refrigerate it for at least 12 hours.
Detailed instructions here.