I assume that people who created the spells just spent a lot of time waving their wand in different movements and ways until something different or unusual occured. But they would have to be careful. Because I imagine that if this was the case, then they would probably damage a lot of things. What I'm trying to get at is that If they where spending all their time waving their wand, then don't you think that they could forget that the movement that they're doing is the same as another spell. Like they could move their wand the same way they would move their wand when they wanted to cast the wingardium leviosa charm, or the muffilito charm? They would probably have to take precautions. But I expect that if they where to create a spell, then they would register it at the ministry. (but of course, they would gave to write the movements that they used in order to cast the spell, or they would have to preform it infrint of ministry workers, so they could see what the spell did, and to see how dangerous it was. After all, it might not even be worthy of a release because it's not good enough. But I assume that if it was good enough, then the ministry would name it and release it to the magical world.