Answer :
here is an idea!
Hi (your friends name)! How are you doing? (after the question mark say what you want to say). (after the period you can say goodbye or something to that effect)
Sincerely, (your name)
tips! before you right your own letter, make sure you make sense and know what you want to say, whether this letter is just practice, or for an actual friend.
hope that helped !!
