Self-taught hard qigong is the Eight Treasures Boxing Gate of South Shaolin, headed by Mo Wendan. He has written two books. Find a second-hand book to read
Fitness qigong recommends intelligent qigong, which not only knows why, but also knows why. It has a complete and effective theoretical system and method, and can learn at a high level without a master. Please search "Tong Ling Station" for details. The information on it is very complete. If you want to get started, please download Concise Intelligent Qigong first. From the point to the surface, you can hold your breath through the top, learn by yourself with three hearts, and practice your breath for three months. If your constitution is sensitive, you should feel angry soon by practicing the method of holding your breath.