When I was little, my mother told me that these were made from corn starch. I first knew that she was lying when I saw that she always used a packet with Chinese all over it, and our corn starch always came from the local American store.
Before the internet, there were only 2 ways to get culinary secrets out of my family... giddiness (in hind sight, it was probably alcohol), and old age (complacency). Even then, you never knew if it was a decoy.
My mother let slip recently that liang fen are actually made from green bean, or mung bean starch. The internet confirms this.
It is whisked into water at a 1:3 - 1:5 ratio depending on how soft you want your final result. I tried a few different consistencies. You put the mixture in a pot and start stirring while the mixture heats to a rumbling boil. It was congeal very quickly, so don't stop stirring.
You then leave it to set for about 2 hours.
I like to eat them with thinly sliced strips of english cucumber, and a sauce made from soy sauce, vinegar, sugar, and the only other hot sauce that we always have other than Sriracha:
The sauce, Lao Gan Ma, literally translates to "old dry mother." My mother makes a mean hot sauce, but the security at the airport has a problem with it. Old Dry Mother ships to your local Chinese grocery store, and you don't have to survive dangerous spice fumes during the cooking process.
I just leave these noodles as a block in the fridge, and whenever I want a quick cold refreshing noodle-y salad, I slice up some noodles and mix up a bowl of liang fen. How fen!