Rowling doesn’t give a lot of information on how wizards invent spells, but it seems likely the process involves some of the following elements:
Many of the spells are based in Latin, though some originate in other languages. I don’t suppose there is any rule that precludes English. My guess is a spell requires language that encapsulates the caster’s intention. Perhaps also, some words/languages are more intrinsically powerful than others. An affinity between incantation and mythology is also likely. Furthermore, I suggest the power of an incantation lies partly in the rhythm of the words. It seems to me Rowling’s spells usually scan to a definite meter, which makes sense. The etymology of “incantation” suggests singing.
The majority of spells come from Latin words or phrases, such as
Expecto Patronum – Latin for “I expect (or await) a guardian”, used to summon a Patronus
Petrificus Totalus – “petra” was the Latin word for rock, and “ficus” was a suffix which meant “to make”. “Totalus” meant “total”, so when put together, they mean “make rock totally”!
Expelliarmus – the English word expel came from the Latin verb “to drive” and the suffix “ex” which meant “out”, so they meant “to drive out”
Lumos – from the Latin word ‘lumen’ which means light
And his brother Nox – the Latin word for ‘night’
However, there was one special spell that had a secret meaning behind it – Sectumsempra of course! Snape was fond of Latin as well so he made a curse by his own from the Latin words “sectum” which meant to be cut and “semper” which meant always. This together combined to form one of the most powerful spells, a spell which one of the books in the series revolved around.
Go Harry Potter!!
Knowledge of wand work.
It’s clear that wands require specific, precise movements to accompany the words. In Philosopher’s Stone, Flitwick emphasizes the necessity of swishing and flicking the wand, as well as correctly using the incantation. As figure skaters once learned to make “compulsory figures”, I think the inventor of a spell must learn a set of wand movements and their effects. He may also try out new wand movements, making an educated guess as to their effects
Knowledge of nonverbal magic
Some spells are/can be cast non verbally, which requires the ability to understand and use nonverbal magic.
We all love Harry Potter. Don't we?
Some daring is necessary to try out invented spells. A competent wizard should have a pretty good idea what will happen, but one never knows until he has actually attempted the spell.
Love Harry Potter. And Love Cinemor