What is proper etiquette for addressing thank you notes?

I would let FH write the note (in his handwriting - you can tell him what to say if you want) and you just sign it along with him.

Alternatively, you can also use something like, "Grandma Lastname" which is a bit more formal.

We each took care of writing the main message to our own sides. So your FH would write "Dear Grandma..." and then you either just sign it or write something short after his message.

I would address it as Grandma [name]. Have FH write the note with both your signatures at the end!

Seriously, let him write it, even if you're dictating -- thank yous don't need to come exclusively from one member of the couple!!

Thanks for the advice!