Step two: Read the instructions online for how to properly cut a latex mattress.
Step three: Completely disregard instructions and start hacking away with a hot knife instead.
In my defense, I didn't find anywhere online that says NOT to do it with a hot knife, but this is what I learned. When you heat latex it does not magically re-harden like plastic does, instead it forms a sticky mess that reeks like a combination of burning kerosene and hashish. The odor is very potent, and does not go away, nor does the stickiness. So yeah, that was a mistake.