There are several styles of martial arts that use a wooden dummy for practice. This is especially the case if you are a student of Wing Chun Kung Fu. Building your own Jong can take a lot of time, or not too much time depending on your experience and the tools you have available.
Jongs can be built from a variety of materials, but I will concentrate on wooden jongs here. The main body of the jong is a cylinder approximately 9 inches in diameter and 5 feet in length. Most dummies are constructed with a single leg and three arms. These represent the body of your opponent and possible lines of attack.
The ends of the arms on the dummy are meant to be the elbow point of your attacker. The single leg represents the forward leg of your opponent as well as your point of attack.
There is no single way to construct a dummy. There are numerous dummy plans available. The most important point is that it be the right height for you and that you practice regularly. It is not the force of your attack that matters so much as that your movement is the proper movement.
That is the whole point of using a dummy. It allows you to practice your movement over and over again so that it is fluid and quick. You should not even have to think. You will simply see or feel the lines of attack and instantly react as though second nature.