OK, the object is to cut out 1/4 of the top barrel as this will be the opening of your smoker. I found it easier to just take duck tape and use it as my outline.
	It works out that 18 inches is 1/4 of the barrels diameter. I just marked with my scribe in 3 place across the barrel and lined up my duck tap. Then put the tape for the outer edge right up against the edge of the barrel, see the above photo. As you see in the photo below I just followed the inside edge of the tape with the saw.
	You've seen the saber saw go so I'll just show you a photo of the finished opening on the top barrel. I've still not mounted the top barrel to the legs yet. I want to finish the vent first, so lets go see that.