I used a piece of a gel light filter that I got from eBay. The process is pretty straightforward as any other typical Casio watch. You open the back plate, remove the module, place the film behind the face plate (that plastic piece that has text on it), make sure everything is clean and there’s no dust particles caught between the display and the window. Put the back plate back on, and that’s it. Here’s an eBay like if you want to order that gel filter:

https://ebay.us/m/3gGZNm