Spell creation

Spellcraft is the highly dangerous and mysterious art of creating new spells. Only witches and wizards with great understanding of magic are known to be able to invent and create spells.

Processes
Not much is known about this form of magic. Most wizards and witches only know it is extremely dangerous and that a lot of people have suffered a great deal of injures and even death, hence few attempt it.

It seems centered very much around Latin. About 90% of all the spells are Latin in different forms; presumably, in order to create spells, it is very important to find the correct Latin pronunciation (as seen in Snape's potion book, wherein line after line had been crossed out, apparently as a result of trial-and-error.)

Behind the scenes

 * It is possible that Gilderoy Lockhart was a Spellcrafter, as he seemed to make several spells (Deboning, Peskipiksi Pesternomi, etc.) up on the spot. This would explain why he did not expect them to work the way they did all of them producing the correct effect (etymologically speaking) bar Peskipiksi Pesternomi.