Words of condolences to a family should begin by addressing the letter to the family by writing something like, "Dear family of," followed by the name of the deceased. Follow this with a direct statement of sympathy for the family, such as saying you wish to express your most sincere sympathies to them for their loss.
The body of the condolence letter should contain more expressions of sympathy. If you are religious, it is appropriate to express that the family is in your thoughts and prayers. If you or the family is Christian, consider quoting verses from Psalms or Corinthians to express that the deceased is now at his spiritual home.
Consider adding a specific fond memory of the deceased, as this can also bring back positive thoughts for the family.
Another possible condolence is to express your empathy for the family's loss, such as writing that it is never easy to lose a parent. You may also write that words are inadequate to express proper sympathy for the loss they are experiencing. Consider adding that the surviving friends and family will always keep the deceased alive in their hearts.
Overall, the condolence letter should be kept sensitive and simple. One possible formula is express your sympathy for their loss, share an amusing memory about the deceased, briefly mention what made the person so unique, and express that you will also dearly miss the person who has passed away.