Just sign as normal,you may add a message prior to your signature,....thinking of you during this difficult time, my prayers are with you, let me know if I can do anything for you!!!!,you have my sympathy and prayers...hope this is helpful
You can sign it: Sincerely or In deepest sympathy or All my sympathy or You are in my prayers, May God be with you, Your friend (s), Thinking of you. There is no set form to follow. Just say whatever feels right to you. The fact that you take the time to purchase (or make) a card is evidence that you care about their loss. It will be appreciated.
Just sign as normal and add something like tell them how sorry you is and tell them that you wish them well!