Step 3: Bottom Cuts
I found that because to the thicker aluminum, the bottles could not be cut easily with a utility knife.
A hack saw works well to make the cuts.
The fixture shown below helped to start the cuts and made sure they were straight around the can. Several turns with the blade pressed against the can started the cut; which was finished by carefully sawing around the outside of the can with a hack saw. (do not try to cut straight through the can as the blade will bend the edges of the can once it breaks through the skin.)
2. Finish the edge of the can to remove any sharp edges or nicks. Any nicks at the edge will initiate a split when the can is stretched.
Place emery cloth (sand paper for metal) on a flat surface. Place the entire edge of the bottle flat on the emery cloth and turn the bottle to evenly smooth the edge.