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".
You can find lists of idiomatic expressions in dictionaries, language learning websites, and books specifically focused on idioms. Online platforms like Cambridge Dictionary, Oxford Learner's Dictionary, and ESL websites like FluentU and ESL Gold provide extensive lists of idiomatic expressions. Reading books or listening to native speakers can also help you learn idiomatic expressions in context.
Cliche's.
Idioms are important in studying English because they reflect the cultural context and nuances of language. Understanding idioms helps learners grasp the subtleties of language usage and improves their overall language proficiency. Additionally, idioms are commonly used in everyday conversation and writing, so familiarity with them is essential for effective communication.
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