ELA5, CSS, Sferrazza

The next thing to do is to think about your audience.

We do not know who is going to pick up and read the letter so that does make it a bit difficult.

Your writing will be a mix between formal and informal. Because this is a more personal letter, you can use shorter sentences and words. You can use contractions and have a friendly tone.

To make it easier to read and feel respectful, do not use slang or text language. 

You need to have proper spelling, punctuation, etc. as that makes the letter easier for someone to read. It also shows respect that you took time to do your best writing for them.