The blade has to fit easily into the neck of the bottle. You can also try using a serrated knife so that it grips the cork better. Exercise caution when using a knife. Make sure not to cut yourself. Carefully work the knife back and forth into the cork, using very little downward pressure. It should go all the way through the cork.

How would you rephrase that briefly using Vietnamese?
Tìm một con dao bỏ túi hoặc dao gọt trái cây. Cắm mũi dao vào nút chai.