Basically make sure there is NO propane left in it.
Lots of tips above & in other threads.
The short version (a series of steps, just one step
Will NOT make the tank safe for cutting):
((Disclaimer: if you do it wrong and blow your head off
don't come yelling at me!))
Open valve, point valve downward & let all the gas out.
Carefully remove valve without making a spark.
Leave tank in direct sunlight for a week OR
Put it on some firewood THEN light the fire (with
a 10 ft match).
Fill tank with water and cut it. Optionally you might
remove the water before cutting, I left it in 'cause
I'm no expert!
Another option before removing valve (as improper
valve removal could make subsequent steps unnecessary)
is to shoot a couple holes in the top part of the tank.
Huh, I COULD be a technical writer after all!