The safest and easiest is spray carb cleaner.
Just spray a very short burst into the carb with the choke open and crank. It may start and stop a few times but it will soon fill the float bowl providing the needle valve isn't stuck closed by gas varnish.
In that event, disconnect the carb gas intake line and spray some carb cleaner directly into the intake to clear the needle valve.
This works every time for me on old tractors, trucks, cars, lawnmowers--you name it.
