It’s believed he developed this system of healing based on a formula he found in ancient Sanskrit Buddhist sutras. Following the formula he went into the mountains and fasted and meditated for 21 days. On the last day he had a vision, which completed his system. Some say that during his vision he received the Reiki symbols that are part of the system. Others say he got the symbols before hand from the Temple where he studied. In either case, the system he developed became an effective tool for healing.
Usui Sensei spent the rest of his life sharing his Reiki system and trained many others how to teach it. Before his death in 1926 he had set up a group called the Usui Shiki Ryoho in Tokyo to continue on with the practice after his death. Through this group many teachers have been trained in Japan.
For my sources see: Recommended Reading