When you look at the carto from the side you see
threads on one end. Use the pipe cutter to cut the
outer housing appx 3/8" of an inch from the
threads. You should be left with the threaded end and
3/8" tube with inner guts sticking out. The part of the outer housing that you cut off can be thrown away.
( or save for other uses ) The 3/8" tube is what goes in the bottom plug to seat the CE2 in
tank.
It goes without saying you should be using steel CE2's, not clearomisers.