Its pretty easy...Just do a search on bokeh shaped lights in youtube everything you need to know is there.
To work out how big you need your shape to be I find It will vary depending on the focal length of the lens used and the aperture used.
Smaller the f numbers the better or the lowest that you can go. Mine was at f2.8 and was on a focal length of 70mm.
so based on the above 70mm divided by F2.8 = 25mm
so your cut shape needs to be no bigger or contained within 25 mm square from the centre of the circle. I made mine slightly smaller than 25mm and it worked for me.
Set your lens to manual focus and turn it until you can see the shape.
Hope that helps