In the case of Shinto, you should write "志," "偲草," or "しのび草" on the front side of the koden-gaeshi and decorate it with white and black or silver mizuhiki tied in a square knot.
神式では「志」「偲草」「しのび草」と表書きし、白黒あるいは双銀の結び切り水引をかける。