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About 1/2 to 3/4 inches (approx. 13mm to 19mm) from the top and bottom, draw a line across the can. It should go about halfway around the can and then finish with another down the middle of the two lines drawn on  the can, to form an "I" (see the image). Make sure that the lines match up on their ends. You will want to open these 'doors' when you use the light.

Summary:
Create a 'door' cut. Cut a hole in the plastic lid to match the size of your light source.