Idioms are common in informal context, but they are found in all registers of English usage.
"There was no breeze. The sea was as flat as a plate-glass window."
Not always. Sometimes idioms are just phrases.
If you are not familiar with idioms, I would not try to specifically write a letter using them. If this is an assignment of some sort, you need to look up some common idioms and think of ways to use them in conversation so you can write a letter.
Try this question for examples of idioms.
One of the most famous idioms about actors is "break a leg". Another common one "he or she has greasepaint in his or her blood".
There is no one list of all idioms. Every language has its own idioms, as do most of the countries. Here's a link to some common examples.
Cliche's.
English-speakers use a lot of idioms! It's a good idea to learn most of the more common ones so you understand what we're saying.
Here is a link to some idioms. You can pick the "non-common" ones for yourself -- some people have heard certain idioms a lot and others have not heard the same ones, so you'll need to decide that part.
Check out this question for idiom examples.
Idioms are common in informal context, but they are found in all registers of English usage.
To provide interesting and vivid conversation.
The most common idiom about cats is "Curiosity killed the cat." A cat has nine lives. It's raining cats and dogs.
K-39 isotope has 20 neutrons
your most favorite person in this world
idioms that you can say