Freeform magic is one of those things that is both enticing to some people, but extremely difficult if nigh-impossible to actually pull off in a way that is satisfying. Despite everything in the game already going via DM fiat, freeform magic requires just even a little bit more.
Regardless I've still always wanted to try it and think it fits well with high fantasy. If you're playing a Sage, add half your level to the roll if you're casting a beneficial, divination, or “sealing the ancient evil” type of spells.
For this system, all magic or “elements” is divided into 10 forms for ease of use. These are equivalent to the forms from Ars Magica.
Beasts (body and mind), Images (perceivable things), Plants, Fire, Water, Air, Earth, Mind, Body (of intelligent creatures), and Prime (magic itself).
To cast a spell, you need a ritual component similar to each form of magic you're using, as well as sympathetic elements to influence them, and finally a fetter to bind your spell to whatever you are casting it upon. Fetters are usually personal effects if cast on a person, a bit of silt or a brick if cast on a location, or an inanimate object itself if cast upon it.
For example, if you wanted to cast a spell on a rival shepards flock to make them stampede and flee you would need a Beast element, a fetter, and something that causes fear (especially to sheep). In this case, something like the wool of one of the sheeps in his flock could work for two categories.
Once your ritual is completed, roll d20 + intelligence mod. Depending on how difficult the effect is, it will be harder and harder to pull off. Ritual spells done this way must be localized and specific, you can't really cast a ritual spell to topple an empire unless you were really high level. Ritual spells often follow the magical path of least resistance, and so there is often a question is the ritual even had an effect at all. In the above example, a wolf might break into the paddock and cause the sheep to flee, which could be caused by the spell or not, nobody knows. Higher level characters could have ritual effects that display vulgar and obvious magic though, such as a ghostly spirit that appears to spook the sheep away instead of a subtle sort of thing.