How do you write a thank you note for memorials after a death?

I'm presuming you either mean one of those small cards that are placed at the burial site, or crematorium, or even a note of passing?

In which case, both sets are pretty much the same.

Either use the pre-given cards from the parted's family and write something heartfelt or significant between you and the parted, even just a thankyou, or goodluck will do, then place it either in a collection point that will come around during the service, or actually by the burial or cremation area.

If it's a note in passing, simply write down what you feel on a piece of paper, or even think it whilst your near the area.

The message will get across on an emotional level either way.