A decent summary of the system is written up on wikipedia, What I'm mostly interested in right now is the five phrases (the aforementioned "aspects") which sum up each character, and can be invoked by the player for re-rolls, bonuses or other effects.
So, a sample character in the book has the following Aspects:
High Concept: Infamous Girl with Sword
Trouble: Tempted by Shiny Things
I've Got Zird's Back
Sucker for a Sob Story
Secret Sister of Baranthar
These aspects are defined during character creation. The default system in FATE is that your first Aspect is your "high concept"; literally just a short phrase describing your initial idea for a character. The second is your "trouble", a sort of long-term flaw or complication that further defines the character. The last three aspects are the "phase trio", aspects derived from a specific collaboration between the players on the group's background/origin story.
The first two aspect choices are perfect, and I foresee using them as-is in Imperialist Hearts. The phase trio, while a really clever idea for forming a rapport between a starting party that might not otherwise have one, is not right for my campaign. Having wildly disparate characters thrown together for the first time against a common enemy should be too fun to be relegated to the backstory. Also, the players will need those Aspect slots to define the difference between their characters' original genres and settings.
I haven't figured out the specific procedure for this part of character generation, but I have a few ideas. Something along the lines of: Genre, Catchphrase, Wild Card. That is, the character has one aspect that describes their original setting/context, one derived from a catchphrase (or other dialog attributed to their character in their source material), and then the last one is free-form to fill in anything the player thinks is important that hasn't yet been covered. Some examples:
High Concept: Wisecracking Rabbit
Trouble: Holds a Grudge
Genre: Looney Tunes
Catchphrase: "Ain't I a Stinker."
Wild Card: Looks Great in Drag
High Concept: Superspy
Trouble: Alcoholic Womanizer
Genre: Cold War Satire
Catchphrase: "Something Something Danger Zone!"
Wild Card: Walking Arms Encyclopedia
This is still very much first impressions/brainstorming. I'll need to reread the FATE Core book another time or two, at least skim the FATE Toolkit, and look around on blogs or forums for examples of FATE variants and gameplay, before I set anything in stone (or even a .doc file). I expect I will write many more posts in this topic as I delve further into the system and adapt it to my needs.