Ok, so first of all, I should point out that this is how I did it and not necessarily the best fit for everyone. It has worked really well our group though. I am using Mage the Awakening.
1st and foremost, remember that no matter how you slice it, its not going to be a perfect fit.
2nd: Most of Paradox has to go. First of all, there is no difference between Vulgar and Coincidental Magic (except of course how easy it is for someone else to tell you are casting magic). There are no chances of a sleeper "Disbelieving" a spell. Dresden casts magic in front of people all the time. In fact, really the only place Paradox remains appropriate is as a penalty for botching a spell, or maybe under Time or Fate Magic (as both of those can involve rearranging the Universe).
3: Ease up on the penalties for Spontaneous Magic (this might be a good idea even with base Mage). Dresden has a few things that might qualify as "Rotes" but a huge percent of the magic he casts is spontaneous and has never been done before.
4: If you do use Rotes, change their die code somewhat so that Gnosis is still apart the casting. Dresden makes a point of how important Raw Magical Strength is to spell casting. For the Record, I rule all spell casting as Arcana+Gnosis, With difficulties ranging from 0 to 10 (in the case of really difficult magic). If the character has spent the XP for a rote, then I reduce that Difficulty by 2.
5: In relation to spontaneous casting, lower the difficulty for Forces even more than the rest, because in the Dresden Verse it's primary function is blowing stuff up (evocation).
6: Since Spontaneous Magic is going to be a lot more prevalent, you will want to either make Mana more plentiful (which is what I Did) or only charge mana for spell casting every so often.
7: Remember (and this is really important) to give out bonus dice to players who give a lot of thought to their magic, such as describing rituals, getting proper components, etc. This is a HUGE part of the Dresden Verse.
8: Two things I recomend making entirely new Mechanics for are Soul Gazes and Third Sight (WOD has something similar, but not close enough). For Soul Gazes, which can usually only happen when both participants are willing, let players describe what the other person sees as long as it makes sense. In some cases, one or both participants could be called on to make willpower rolls to avoid being shaken or even paralyzed by what they've seen.
For Third Sight, it's basically about how long the character can stand to keep their Eye open. The longer they do, the more information they can gleam (the whole "You never forget" is mostly a role playing thing, but you could call on it to create a derangement). 0 Success yield nothing, the character was too overwhelmed by the flood of information, 1 success would be very basic, 2 successes would give more specific information and so on. In this instance, the Occaltation Advantage could be used to subtract dice from the willpower role.
For example, Markus Cheerful is using Third Sight on Emilia Larson, whom he suspects is up to no good. His Willpower is 6. Larson has 1 dot of Occaltation, so Cheerful Roles 5 dice. He gets one success, and for a moment sees a shadow pass over Larson's face. If he had gotten Three successes, he might have fully seen the Dark Aura surrounding Larson, and the tormented spirits chained to her.
9. Finally, there's the issue of magic breaking technology. I handle it like so. Find the characters Highest Arcana (or use Gnosis if you prefer). A number of times per session equal to the characters rank in that Arcana, the GM can randomly Break any piece of technology with a few meters of the Wizard. However, the player may also break technology in the same fashion, with the same number of uses. This represents how in the novel, Dresden's unfortunate relationship with technology both helps and hinders him. It should be noted that this ability is used by the player, not the character. As far as the character is concerned, he just got a luck break.
Certain pieces of technology that are either older or especially well built (such as a WWII Pistol or a Sturdy Volvo station-wagon) may require two or more uses of the ability, and certain particularly fragile pieces (like a laptop running windows) might only require half a use.
10: If you are really set on being true to the Dresden Verse, you may want to remove Death and Time Magic. Death because in Dresden Verse, Necromancy is sometimes described as being more powerful than any other form of magic (open to debate I suppose) and Time because the only character we've run into so far with any time manipulation abilities is the Gate Keeper and his seem very minor.
There is totally more, but I have to go again. I'll post the rest later. I hope it helps.
