A well structured condolence or sympathy letter usually has 7 parts.

Step 1. Acknowledge the Loss. 

      As a starting point you need to acknowledge the loss. 
Examples could be:
I was sorry to hear of your recent bereavement.
I was very upset to hear of your loss.
I wanted to write and tell you how sorry I am for your loss.
I was so sorry to hear about the tragic circumstances surrounding X's death.
I was terribly sorry to learn about the death of X.
I was saddened to hear of X's death.
It was with a great sense of loss that I learned of X's death.
I was so sad to hear of X's sudden death.
I am writing on behalf of all of X's friends to express our sympathy at your sad loss. 

Step 2. Express Your Sympathy. 

Examples could be:
Please accept my sympathy for your sad loss. 
I want to express my heartfelt sympathy.
X was such a special person that no words are adequate.
X was such a special person and it must be so difficult to come to terms with the fact he/she died so suddenly.
May it comfort you to know that so many people care and are thinking of you and your family at this time.
May the love of family and friends comfort you.
There are no words to express our heartfelt sympathy to you and your family.
While there is nothing I can say that will ease your loss, I wanted you to know that I am grieving and thinking of you all. 
 
Step 3.  Note the Special Qualities of the Deceased.

     Think about the special qualities you admired or appreciated about the deceased. Examples could be:

X was such a wonderful person, and so many people will miss her.
X was such a creative person, and I am so  sorry he/she has died.
X was a valued member of the team and contributed to the organization in many ways.

Condolence Letters and Sympathy Poems

Step 4.  Recount a Memory About the Deceased.

     You could talk about how the deceased touched your life or if appropriate a funny story may be included.

People tell me how much they valued X's friendship.
X bought pleasure to everyone he/she met and will be sadly missed.
He/she often spoke of his/her family with affection.

Step 5. Offer Assistance.

But don't make offers you can't fulfill.

If there is anything I/we can do, I/we are only a phone call away.
I am always here to talk if you would like. I'll get touch soon to see if I can make myself useful in anyway.
If there is anything I can do such as ??? please let me know.
Please call if there is anything I can do.
Thursday is a free day for me to come over and help.....   

Step 6.  Close with a thoughtful word or phrase.

Our thoughts are with you at this difficult time.
X will be sadly missed by everyone at work.
May the love of family and friends comfort you.

Step 7. The Ending and Sign Off.

With love and sympathy,
Sincerely,
Your friend,
Love,
Affectionately yours,
Yours,
Very sincerely
With my deepest sympathy,
My fond respects to you and yours.