To write condolences for the loss of a father, write from your heart and offer to help the recipient in a practical way. If you knew the deceased, share a positive memory of him.
Begin the letter with an appropriate greeting, based on the recipient and your relationship with him. If the recipient is a friend of yours, an appropriate greeting is "My Dear Friend," followed by his name. If the card is intended for multiple family members, an appropriate greeting is "Dear Family," with the family's last name after the word "dear."
It can be difficult to find the right words for a condolence letter, particularly when beginning. Write sincerely to express what the deceased meant to you and the sympathy you feel for the recipient. Sharing a story about the deceased, or writing the ways in which you admired him, can comfort the recipient.
Ways to help the recipient include doing household chores for him, providing child care or dropping off meals. Offer to help in a specific way, then do it without being asked. Don't offer financial help.
Respect the recipient's religious beliefs, and avoid mentioning anything that focuses on the tragedy of the situation. If you're worried about what to write, keep the letter short.