Having just run Dresden Files RPG at a convention, here are my suggestions.

Have handouts for the players giving a (brief) outline of the FATE system covering the following points:
Skills and difficulties
Aspects and Fate points
How to perform actions/checks
Explanation of Mortal Stunts and Supernatural Powers

I found that I could generally explain how to play the Dresden Files RPG in just a few minutes, as the basic rules are easy and intuitive.

I would also recommend providing Pre-Gen characters, but as others have suggested, leave some areas blank for the players themselves to finish and also customize.  This lets the players have much of the work of character creation done for them, yet still give them some room to add their own 'flavor' to the character, and it will also allow the player to fit the character into whatever their concept of the character should be.

For the Pre-Gens here are my recommendations:
1.  Provide the High Concept, Trouble, Background and Rising Action aspects, but leave the First Story aspects empty, and have them filled in during game play.
2.  Chose the skill tree to use, but leave some slots open for players to select (or rearrange) skills.  In other words, a Feet in the Water character (20 Skill pts, max of Great) can have 2@ each of Great, Good, Fair and Average, and you might only fill in 1 each of the Great, Good, Fair and Average with the slots left empty for the player to choose.
3.  Leave at least half the Refresh open for the Player to choose on their own.  This is where a good deal of the favour of a character can come from.  Additionally, you might have players that really want to/not want to play casters.  Leaving several Refresh open allows the players to choose what to do there.

As an example, I had a pair of one-shots which I ran which took place at the Grand Opening of a Harry Potter-themed cafe & bookshop called the Leaky Cauldron.  Running with the Harry Potter theme, the aspects for the characters as well as some of the skills were all based off some of the characters from the books or movies.  By allowing a fair amount of customization for the players, I ran the same thing for two completely different groups and entirely different outcomes.

My first post, so please, be gentle...

Having taken a quick read through of the forum, this post struck me so I thought I'd toss my $0.02 in.

The first thing which came to my mind, is that a "magic" gun which can give fired rounds an elemental effect is a bit over the top uber-powerful.  Secondly, such an item would pretty much be a First Law breaker, at least in human hands.

For starters, a decision would need to be made as to how such a gun would "work".  Does the weapon actually enchant the round fired?  Or does it just replace any round shot with an elemental effect?  Given the various problems with allowing a "magic" item to enchant other items, I would expect that it would be more reasonable to allow the "gun" just shot elemental effects instead of bullets.

This would make the gun not unlike Harry's various force rings, the lethal use of which upon another human being is a clear First Law violation.  Additionally, the force rings are quite limited in terms of usage, both in the books and the RPG.  

So, at present to get a "gun" which fires an elemental effect of strength 2 (standard pistol is Weapon:2) able to be used once per session, it would require the following:
a minimum Lore of Average (+1)
an enchanted item slot
either Ritual or Thaumaturgy: with the Crafting-Strength specialization

Now, if the "gun" is to supposed to be able to be used by others, then the creator needs to either have an additional rank in Lore (min of Fair +2), or a Refinement of +1 to the Crafting-Strength specialization, or an additional enchanted item slot is required.  Similar requirements exist for the "gun" to have greater strength/power

If additional per session uses of the "gun" are desired, then a combination of additional ranks in Lore, enchanted item slots, or Crafting-Frequency Refinements are required.

Having the "gun" be able to use additional elements beyond the first one would also require additional enchanted item slots, one for each additional element desired.

In terms of someone from the Nevernever that might be able to fashion such an item, a svartalf likely could, even with using iron.  It would just be costly.

Lastly, a Knight of the Cross does not kill human mortals with magic, even using a Sword of the Cross.  If a Sword was used to kill a human mortal, it is still a sword, magic is not required for it to deliver a killing strike.  A Sword of the Cross itself is not really so much an enchanted item as it is an Item of Power, being in essence a Focus Item for the power (Faith) of the wielder, and/or acting as a focus for the Faith of people in the world.  Not unlike what made the theft of the Shroud of Turin such a concern.  It was less important whether or not the Shroud was "real", the fact that so many people believed it was/is "real" and for such a long time, potentially made the relic a focus/source of power.