Years ago, a tremble was easy to write, you simply put a squiggly line around the shoulders, the head, or other thing which was trembling.
Here is a sign I have run across in a video which I don't know how to write with the current ISWA.
The upper hand in a rose-petal shape presses on the lower hand and trembles. It's the sign for "gelatin" or "jello". I chose what looked like the old "tremble" shape but I don't think I chose right.
The upper hand does not turn over,
it wiggles against the lower hand. The wrist may move in a tiny circle, but it's not clear what it does to me.