How To Write Message Of Sympathy?

I would write a simple note in a blank card and drop it off with a dessert, or a pot of live pretty flowers.  If you can't do this just mail the card as soon as you can.  I think people need to know they are cared for in their time of need.
I'd write something like this:
'I just wanted to let you know how deeply concerned I am for you and yours. My prayers are with you. If there is anything you could use my help doing please call me at any time, (555-5555) Just know you've got a friend in me, and I am here for you.'

Dear Mary,
I wanted to contact you as soon as I learned that Don had passed away.  I am so sorry that you are going through this heart break.  Don was loved and admire by so many people.  Don had such a  positive influence on my life; I will miss him tremendously.  I would like to express my sympathy and let you know that you and your family will be in my prayers.  
Sincerely,
James Lewis

At a time like this words are not enough, but wanted you to know that you can call on me anytime, and I will cherish the time God has allowed us with, NAME, here on earth, please feel free to call on me when needed, I'm here.......