I handled something like that - the character was made an apprentice and slowly bought what was missing from the wizard template.
In Summer Knight they mention an ordeal - maybe it's something that you get after you pass the test and are seen as powerful enough to join the council. Note that Harry skipped his - they thought that killing an experience wizard in self defense was enough of an ordeal.
I asked the player if he wanted a downtime "You went away an apprentice and came back a wizard" bit or if he wanted to play it out, and if he wanted to play it out did he want a chance to fail. He decided to play it and risk failing.
So I did up a whole ceremony and the next time the group wanted to check out his mentor's library I sprang it on them. When he arrived at the place where it was going to happen he was handled a brown robe and told to take a ritual bath - and something along the lines of "I assume you know how to use a face cloth so you can handle this yourself". That was the first part of the ordeal he could fail. I mean, if you say you can't give yourself a ritually cleansing bath then how can you call yourself a wizard?
Then I told him that he could start making declarations for his next spell. The rest of the group chimed in with suggestions what he could declare and what skill to use for it. For example, the bathroom was described as candle lit with scented water in the bath and a nice smelling incense filling the room - and I thought he might declare "The Lighting Set the Mood", "Incense Filled Lungs", "Centred From His Bath", "Relaxed" - that sort of thing, using Lore, Discipline, Conviction, and any other skill he could justify. He didn't pick the aspects I thought he might, but he started declaring things and we wrote down everything he declared.
Sometime during the bath his clothing went missing from the room, leaving only the robe and the towels when he emerged from the tub. (Another choice: would he wear towels under his robe. He went with just the robe and made a Lore declaration "Dressed for the Part".) Then as he left he found the hallway outside the bathroom had been lit. Candles of one colour burned on the left hand side of the door, candles of another colour burned on the other. He could pick a path, or just follow one back to the stairs and leave (leaving would be a failure). After he picked he asked if there was any meaning to the colour and when I shrugged there was a Lore declaration for "Following the <colour name> path".
As he walked down the hallway there were other things he could use for inspiration (including things that I hadn't meant to be used; some of the basic descriptions were used to make declaration) until he came to a maze. Then he had to choose if he would follow the left hand wall or the right hand wall to find the centre. When he got to the centre he was able to declare a few more things.
Leaving the maze he came to a stairway - and could decided to go up or down (another choice and another thing to declare off of). He had a few more choices and ended up at the basement stairs. There were no lights in the stairway and a door at the bottom of the stairs. He declared "Descending into the Darkness" and made his way down.
When he opened the door we switched back to the other characters, who were having brandy and sherries in the basement with members of the White Council. Note: they would been having drinks wherever he showed up - the White Council being better able to guess his movements than I was. By this point he'd been give the chance to make about 30 declaration but the dice had not been his friend on many of them. Oh, and he had also been given the chance to cast spells to make his way easier - without being warned that if he cast any he would be using up all his prepared declarations.
He was told that there was another candidate - and a Brown Robe that was lying on the floor was pointed to. Then a door was pointed to and he was told "Two Brown Robes enter, one White Robe leaves". He had choice, to leave his robe or take it with him, and used the choice to make a last declaration - "Giving it all up for Magic" (or something like that). In the room waited a man around his own age wearing boxer shorts who laughed and sneered something about "Didn't you have the brains to conjure something to wear? This is going to be so easy!" which led to a bit of a sidetrack as the group debated how many steps it would take to conjure boxer shorts and if he should waste an action or two to do so. Note: if he had wasted all those declaration conjuring clothes he would have failed - being too narrow sighted to make it as a wizard (and he wouldn't have been able to pass the test).
He decided not to bother (and tried for a Presence based declaration of "I've got nothing to be ashamed of") and I announced combat was starting and the other guy was swinging his hand around and... Wait, we should do initiative here. He's got identical stats to you so let's roll off to see who goes first. Luckily the player rolled +++[] so he didn't think it was odd when I rolled and quickly scooped up the dice and said he won.
Then there was his action. He asked others for advice - shield first or strike first? The discussion went on for a while I told them that from the way the other guy was moving it was clear that he was going to attack. When asked if it looked like there he had any defenses I shrugged and said "Only the boxer shorts".
Finally the player decide for an attack that could be non-lethal and hopefully take out the other guy in one go. As he started debating how many declarations to use, I pointed out that they could only be used for his NEXT spell and yes, if he had conjured the boxers he would have conjured some pretty fantastic underwear.
He had about 14 aspects to tag, so he tagged them and three of his aspects from his sheet. That meant that his attack spell was 34 + plus conviction + successes from the targeting. And yes, it was huge amount of damage handed out...
Which was when I informed him that there was only one person in the room - himself (hey, he never used his sight!) and all that damage was hitting him. It was going to be a takeout, but he could decide if he wanted to use his consequences. If he filled all of his consequences - including the extreme one - then he would have only X stress left after his track was filled and that I'd be basing the results on how much excess stress was left. The normal consequences were filled in quickly (well, he decided to take them and I said we would work out the details afterwards) but that extreme.... Long agonizing bit over that, but finally he decided to use it.
At which point I said it was time to deal with consequences. It went something like: "Now let's see, for the extreme we have to modify one of your aspects. See your High Concept Aspect of 'Potential Comic Book Inspired Wizard'? Erase the word 'Potential'. Congrats, you passed."
Then we went over the various points where he could have failed. Not being able to declare enough aspects - a wizard has to be imaginative and use what he has on hand. Making enough declarations but dribbling them away on a series of small spells - a wizard needs to be able to conserve his powers, especially when he knows that there is a challenge coming. Not acting decisively when faced with an opponent that needed to be taken out. Not being willing to give it all (including his extreme consequence) for magic.
And when we finished this, we had a list of 14 phrases that he had come up via declarations - words that described what sort of a wizard he would be.
All in all I think it went well - even the players who weren't getting the face time enjoyed themselves. Would everyone had enjoyed it if he had failed? Who knows? But he wanted a chance to fail so I gave it to him.
Hope this helps
Richard