hi noxnightly!
Generally, a spell “fails” if it produces an unintended effect. When players aren’t under pressure, they’re probably not going to fail spells - experimenting and problem solving together is intended gameplay. When a player has to draw under time pressure or with their eyes closed, it’s more difficult. A piece of a spell could be drawn too big, or outside of the circle, or in the wrong shape. It’s up to the GM to interpret the mistake, which can lead to meaningful narrative consequences.
Making magic doesn’t require a dice roll to succeed, but sometimes the actions around making magic require a roll. For example, making an arrow out of water doesn’t require a roll, but hitting a paper fluttering in the breeze with that arrow probably would require a roll. Mixed successes/failed rolls around magic can also lead to interesting consequences.
I’ll also note that even correct spells can lead to interesting complications. Once, to get across a canyon, my playtesters put a flying spell on a log and flew it across - but they didn’t think about how to stop it. So they were rocketing through the air at high speeds and had to figure out how to get down safely. It was a fun challenge!
Hope this helps! :)