Okay, so magecraft itself is something I've always had trouble completely understanding in the Type Moon verse. From what I know it's covered extensively outside of Fate and Kara no Kyoukai, and due to that being where most of my experience lies what I know is limited.
For the most part though, it is very complicated and simple at the same time. Me explaining things may confuse things even more so may I suggest checking out
this and
this link for explanations and example. However, what you need to know is that for the most part, as long as what you are doing doesn't create something from absolutely nothing (ie magical energy) and can exist or already exists (ie your ice) you should be good to get creative. Sorry for the lack of my usual help here. Type moon is super in depth with everything so it's hard to grasp it all without first hand knowledge, haha.