What are the simple and easy-to-learn breathing methods?

Breathing and exhaling is an internal exercise method to exercise internal organs by breathing, exhaling and exhaling. This method can be divided into "forward breathing" and "reverse breathing". According to the usual breathing habits, one belly is bulging and the other is flat, which is called smooth breathing; On the contrary, the tummy bulges and the tummy collapses, which is called reverse breathing.