It's always awkward to write a sympathy letter, but getting it right is quite important. At a time when someone has died, sensitivity in writing style is essential. Before taking pen to paper, think about the person who has died and the person to whom the letter is to be sent. This isn't a time for glib writing. It's a time to put into words the message of condolence in a manner which reaches someone who may actually need your support.
Begin the letter with the normal introduction of Dear.......... , Then think of how to approach the subject. Words which are the most hackneyed in a situation like this often come over as insincere. Try to be a little more original than merely saying “I am sorry for your loss.” This may seem a little harsh, but chances are everyone said the same thing, and saying it too much makes it relatively meaningless. The idea of a condolence letter is to reach out to that person when they need you the most and to say something a little more meaningful.
Perhaps begin with telling them about the memories you all shared. Say how you will miss the deceased and that your sadness at their loss will never get in the way of you being there for them if they need you to be. Their loss isn't a normal event in their day to day life. It's huge. Sometimes, stepping away from the norm and recognizing this as such an important event is worth it, as they may actually feel someone is sincere enough to care.
Let them know that you are there for them. Let them also know that you see them as a friend, and that your friendship with them matters. It's strange what people don't say in times of loss, and those unsaid words can't be said again. This is a chance to help that person with a very sad event in their lives and to show your support.
In the letter, never mention other deaths. People have a bad habit of bringing up other people's deaths when they greet someone who has lost someone. This doesn't help them, and in fact reinforces negativity in them. This is their loss. Don't demean it by equating it to other deaths. Do mention positive things, perhaps small memories about quirks you will remember their loved one for. They will be thinking of that person as well, and you get into their thoughts better by being on the same wavelength as them.
Keep the letter relatively brief but always sincere. Sign it as a friend, and once you have posted it, remember that it doesn't stop there. Follow it up with a phone call, and let them know you are sincerely there should they need you, as chances are they won't phone, since they feel isolated by the events which have taken place.
The manner in which you word condolences matters more than you think. Think about birthdays and Christmas, and those people who send out insincere greetings that say nothing, and once you do, you will also understand that the letters that say nothing or the senders who rely on shop bought cards to say it instead of writing what they feel miss out on the opportunity to acknowledge the importance of the event your friend may already find hard to talk about. This is their loss, and you can help them on the road to recovery if you write a letter which takes time, is well thought out, and doesn't forget the purpose of writing.