How to write Informal Letters
When do you write informal letters?

You write informal letter to someone you know, like a friend or relatives. The language is informal and it doesn't matter if you use shortened words or slang. You can use everyday vocabulary that is familiar for you both.

What do you include in an informal letter?

You write your address (optional)
Date

Useful Words and phrases for Informal letters

You can start the letter using a friendly term such as 
   "Dear Dave"
   "Hello Lisa"
   "Hi Jenny"

You can also use a nickname familiar to you both

Finish the letter using a friendly ending such as 
   "Love, Sue"
   "See you soon, Paul" 
   "Write back soon, John"

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 Sample 1 of an informal letter Sample 2 of an informal letter

Source:
http://lln.runshaw.ac.uk/resources/abe/english/letterwriting.pdf