"While it’s theoretically possible to create a
spell which would allow one to fly, the ability
to fly (whether on a broomstick, a carpet, or
some other conveyance) does not come with
an instruction manual, and thus does not come
with the expertise to control the ability once it’s
available. Most wizards avoid trying, and those
who do usually quickly discover what the human
body was not meant to do." (YS283)
As with any spell, you first have to come up with a way to describe the effect your trying to get. Are you using wind to 'blow' you upward, like Storm? This may seem like a great idea until you consider that a wind strong enough to lift an adult into the air is not going to be gentle about it. Imagine sticking your head out the window of an jet while in flight. (But only imagine it, please, don't actually try it!) Or perhaps you could use fire to give yourself a booster rocket effect, like Ironman? Even if you assume that a human capable of such fine control of three seperate thrusts (intensity and directional vector for each), you still hit the problem that you're probably talking about three spells working concurrently to emulate it.
Also, what happens when the spell runs out? Evocations don't last very long, and if you're 100 feet up and moving fast when your booster spell dies, you're likely to follow suit shortly. Well, why not use Thaumaturgy? Well, Thaumaturgy is designed to create strong, steady flows of magic, which would not respond at all well to the continuous stream of fine corrections necessary to controlling flight.
All in all, the most workable solution seems to be the use of Transformation to grant actual wings. You can get away with Thaumaturgy, because you're really using the magic to give you the wings and the strength to use them -- which stays constant for the spell duration. Even so, though, you have to learn what birds get from instinct built over millenia. And a mistake -- perhaps a bone thinned out a bit too much in order to reduce weight, or a muscle not quite strong enough to withstand the strain of flight -- could be very, very constly. And humans should probably begin the learning curve with Flight skill at zero. The trick is to survive training that skill up enough to be a competant flier.
That said, it's really a question of how your group wants to handle it. You could just say that the above methods work fine, and that part of the magic smooths out the control issues somehow. You know, cuz it's magic, right? Up to you. If you do it this way, then I'd imagine you could stat it as an a spell to allow Athletics to accomplish the impossible ... specifically, to allow you to travel without a surface. So a 6-shift Evocation would allow you to fly with the same speed as you'd run with an Athletics skill of 6 (but without having to deal with ground obstacles). For one exchange, though -- you'd likely want to allocate a few shifts for duration. Or you could use Thaumaturgy to accomplish the same, making it easier to get a longer duration and/or higher speeds.
