Be direct about the reason for the card. A comment like, “I heard about your father, and I just wanted to say how sorry I am,” works. You could also try something more formal like, “I was deeply saddened at the news regarding your father.”
Briefly mention your memories of the deceased, or if the condolence is not regarding a death, write something encouraging, such as, “Once you get better, we’ll have to take a trip together.”
Close the note with something that shows your feelings, such as “With deepest affection,” or “Thinking of you.” It may also be appropriate to include a short encouraging quotation from an author she loves, or a line from a poem.
Include your full name, especially if you have not seen the recipient in some time. This prevents confusion regarding where the note came from.
Avoid writing a long treatise on what has happened since you last met. This is not the appropriate place to catch up. Also stay away from phrases like “I know how you feel.” This sounds trite, and the likelihood is that you do not know exactly how he feels.
Write the card yourself, preferably in blue or black ink. Do not type it, or use a stamp for your signature. This includes addressing the envelope. Handwriting everything gives a more personal touch to your card.