I can't stress how important it is to do this stage safely. The gas bottle must be completely devoid of combustible gas.
If you don't do this stage properly... well... gas + air + angle grinder = hospital/morgue
Even if the bottle is empty, it'll still have residual gas inside which must be purged.
There are various methods for emptying bottles and removing the valve. Have a look on the net, you'll find methods involving using spanners and scaffolding poles to unscrew the valve. Lump hammers to smash it off. Or, as I did, make a hole and fill it with water.
Lots of suggestions here.http://www.mig-welding.co.uk/forum/showthread.php?t=9210
For mine, the bottle was empty to start with. Nevertheless, I first used a screwdriver to wedge the valve open. I left it in the garden for a few hours to be sure.
I then very slowly drilled a hole through the brass valve on the top of the bottle.
The bottle was then filled with water. I drilled a bigger hole in the top. Emptied the bottle and then left it upside down to completely drain.
I then forgot about the project of a couple of months.