system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Snape invented many spells under the name of the Half Blood Prince. How did he invent new spells? What's the spell-crafting process?

Bonus question, if anyone can talk me through the process Snape went through creating Sectumsempra, let us know? I can only imagine the first tests he conducted were truly horrifying!
gpt
The evidence points to magic itself as something that you can train, like a dog. You keep the desired effect in mind and keep trying until you get a result. Then you associate the result with words and gestures until you have a spell that anyone can do with practice. 

Remember when Harry vanished that pane of glass in the London Zoo? If he wanted to recreate that he'd start with recalling exactly what he was feeling at that moment. Once he could vanish glass on command he'd add words and gestures ("Evanescet cum silicate!") until he'd trained them into the local magic field, or whatever it is that retains these things. 

Again, the spell creation process goes from (1) unconscious magic to (2) wandless, nonverbal magic and finally ends with (3) a standardized spell.