Flare guns explained - if you find a flare gun then open your map up and look in the bottom right corner. There will be a crate symbol and a vehicle symbol with a number next to them. If you find yourself in the blue zone and need a vehicle then check map and make sure there is a number next to the vehicle symbol, if there is then fire the flare straight up and a vehicle will drop on the flare. You will not get a vehicle if you are inside the current circle or if there are none available (it will say zero on the map screen).
If it's a care package you want then again check the availability of carepackage crates on map, if there is one available then you must be inside the current circle and not in the blue zone, again fire the flare up and a career package will drop.
Hope this helps as it is a bit confusing.