As I said in the previous post, any magic user can create a spell. It's not rocket science - it's manipulation of energy using symbols and non-obvious (that is, occult) relationships.
Let's start by differentiating between the types of spells I'm going to talk about.
The first is the type of, well, common spellcraft done by magic users. You have a need, you work up a spell, you do it, it's over. You may do the same spell daily for a full cycle of the moon for some purpose, you may do it every now and then over years or a lifetime. But every time you do it you do it anew from start to finish. To keep the Harry Potter references going - this presumably is what the Weasley twins do: They buy the basic cheap plastic toys thru' Oriental Trading Company like the rest of us (what, you don't order a gross of lightsticks before going to a Pagan festival - or a rave - or for your Survival Retreat?) and then do a spell they created on them to add the hoodoo they do. I'll grant that the edible ones are potions work, and different than straight spellwork.
The second type is what stands behind the 'wand work' magic that gets all the attention everywhere. You say a word or two or three - maybe in Latin or pseudo-Latin - and wave your wand in just the right way while concentrating, and is what Hermione can learn and execute. But NO ONE in the Harry Potter books seems to be creating these spells any more. You never see the adult Weasleys learning new spells, or talking about some hot new spell on the market. I'll talk about making this type, too - minus the wand worship part. And usually minus the pseudo Latin part.
There's no reason you shouldn't be creating both types of spells.
So, coming up, posts on creating both types.
And away we go.....