The only way to save fuel with a manual transmission is to shift into neutral and drive the car while it is in motion.
That’s actually both uneconomical and dangerous. 1. because the engine and wheel are disconnected, you lose engine braking, which means you have less braking power and your brake wears out faster. 2. when you go into neutral, the engine quickly goes to idle, which burns fuel, but if you’re in gear there won’t be any fuel into the system because the engine doesn’t need any fuel to rev because of inertia of the whole car.