Today turn on the gas cylinder bottle (to open cylinder valve for gas supply), turn the knob handwheel anti-clockwise. Do not open hard against the stop. Also, make sure the regulator is fitted firmly on the top, depending on the type of gas bottle shape or size.
If you have more than one gas bottle cylinders, be sure to turn both of them off (as with the POL propane gas bottles). To turn off gas cylinder bottle, rotate the handwheel, on the top, clockwise. Close by using your hand only, until it is firmly closed. Do not use tools to fasten the gas bottle regulator.
For gas cooker, barbecues and camping gas - turn off all the burner control after you have turned off the gas cylinder bottle regulator. Note, this applies to POL valves of Calor-gas and Flogas. Procedure may be different for other valves in other countries.
The reality is that gas burner valves and regulator connection can leak. This is especially true as they wear with age. The leaks can be very small and unnoticeable, especially at the hose end, clipped to the regulator position. However, if you don't close the gas cylinder bottle regulator valve, the hose connected to the appliance, overtime, get full and can cause breakouts or high flow capacity when burner is turned on for cooking. LPG will cause natural rubber and some plastics to detolerate. This is why only hoses and other equipments specifically designed for LPG should be used. It is more safer to turn off the gas bottle regulator first before turning on the burners. Turning off gas cylinder bottle first also uses up all the LPG gas leftovers in the gas hose pipe. So, you are safely storing your gas bottle with no remnant of gas remaining in the unit.
If your gas cylinder bottle sits for days or weeks between uses, without uplugging it from the appliance being fuelled, a small leaks can add up and accumulate to areas, creating an ignition hazard. This is why you should never store a gas bottle indoors. If you store gas cylinder bottles correctly, in a well ventilated outdoor location, the gas from a very slow break, would mostly likely just dissipates.
The best practice is to turn off gas bottle regulator valve first, then you go ahead to turn off LPG appliance being fuelled. This eliminates the possibility of leaks from all parts of the burner and regulator connection while in storage. In a perfectly engineered world, everything on your gas bottle would seal perfectly. Check if the gas cylinder bottle was delivered with the company seal and safety cap intact. If you are not sure about usage, please ask the delivery person for a demonstration.