How to use 如 to make comparisons or analogies I see this come up sometimes in idioms, and I find it easy to understand but not easy to use(1) Is it used to spoken comparisons or analogies? Or is it only for literary, formal, fixed expression?  If it can be used to make new expressions, (2) What is the proper syntax?  (3) Can both adjectives and adverbs be used?
It is more of a literary/formal expression; It's not really colloquial. 

I'm not sure exactly what you mean by making "fresh" analogies. If you mean "Can you make new formal/literary expressions when writing literature?"- the answer is yes. 如 is not bound to any fixed idioms/expressions. With that said, it helps to learn the root meaning of the word so you can use it how you want:

如 itself means "as","as if"


Take the following usage in a "fresh" expression:

天上的星星**如**无数只眼睛，在一眨一眨。
(Tiānshàng de xīngxīng rú wúshù zhī yǎnjīng, zài yī zhǎ yī zhǎ.)

The stars in the skies are **as if** there are countless blinking eyes. 




And 如, as you know, is also used in several fixed word pairs:

如果: if
如何: how
如此：if (more formal)


Hope this helps.