Yesterday I was given there of these to start my first project

As you can see I've take out the valve to let the gas out ,I read somewhere that you fill it with water empty it out and fill again
And leave the water in to cut it out the door with a disc cutter or jig saw ,my second question is when you get around to putting the flue
On has it got to be in the middle on the top or can you put it to to one side on top ,any tips would be a big help thanks

As you can see I've take out the valve to let the gas out ,I read somewhere that you fill it with water empty it out and fill again
And leave the water in to cut it out the door with a disc cutter or jig saw ,my second question is when you get around to putting the flue
On has it got to be in the middle on the top or can you put it to to one side on top ,any tips would be a big help thanks