Made of: (làm) bằng

You know bằng for the thing you use: đi bằng xe buýt, viết bằng tiếng Việt. The same word says what something is made of. Put the thing first, then bằng and the material: Cái bàn này bằng gỗ. This table is made of wood.

Làm bằng, "made of", says the same a little more fully, and it's just as common: Cái túi này làm bằng da. This bag is made of leather. No là goes in front; bằng already means "is made of".

To ask, put gì where the material goes: Cái này làm bằng gì? What's this made of? For food, and other things made from an ingredient, you'll also hear làm từ (made from): Bánh này làm từ bột gạo. This cake is made from rice flour.

When you simply name a thing, the material often follows the noun with no bằng at all: ghế nhựa (a plastic chair), nhà gỗ (a wooden house). Use bằng when the material is the point, because you're asking about it or telling someone.

Don't confuse it with bằng after an adjective, which compares: cao bằng nhau, the same height.