I used a thin piece of cardboard, cut in a circle just larger than the UV filter on my 50mm f1.8 lens (I’m used to getting some sweet bokeh out of this lens).
Next, I scribbled a star onto the cardboard circle and carved it out with a craft knife. The cardboard sits on top of the UV filter I have permanently attached to the lens to protect from dust.
To keep the cardboard in place, I put clear plastic wrap over it; held fast by a rubber band which fits snugly behind the UV filter and leaves the focusing ring free.
This is what my camera looks like with it’s makeshift filter:
The picture is just fairy lights on my bed headboard, but next will be streetlights, glitter(!), light reflected in water, coloured lights, highlights in almost anything…