For an expert job.
I tape construction paper to the fuselage (tape it back far enough so you can slip the cowl underneath without taking it off)
cut the needed holes in the construction paper fixing any mistakes as you go. Remove the engine and slide the cowl underneath, mark the holes with a fine tip marker.
Remove cowl and grind away as needed. I use a small sanding drum and a small stone as needed with my Dremmel.