Actor speaks lines -> MAGIC HAPPENS -> finished film... neither Garage Band nor FCP will get you there.
ADR, in so far as the limited occasions where I have done this... and those who have done it professionally can speak in more detail... goes like this:
Make an exact transcript of the on screen utterances of the actor's dialog which is to be replaced with the ADR version.
Bring Actor into a 'studio' setup, where a microphone and audio recorder are available, essentially the same conceptual setup for 'production' sound, but in a more controlled environment.
Have the actor review their onscreen performance, via a 'looped' clip of the scene, cut down to essentially just the dialog in question.
Record a number of 'takes', where the actor attempts to match their previous performance, in terms of lip timing.
You can experiment with the following... actor watches video loop, while listening to the on screen performance, just listen to the audio loop, or some other way to have the actor get as close to
the original in terms of timing, 'emotional content', etc.
Take the recordings made in this session, and bring them in to... well here's where the audio pros will suggest Pro Tools... since I don't have Pro Tools, I would use Premiere, since it is now capable of editing sound down to the sound sample rate of 48K, and would look at Audition, the Adobe 'DAW' offering these days. I only have the evaluation copy, and it is due to expire soon...
Select the 'best' ADR recording that exhibits a close match to lips, and gives the desired emotional tone, if possible. Editing may be required to move individual words closer to the lips moving.
Then add effects to the result to give it a 'it was recorded in that space' sound.
Do this until it is 'good enough'...
I don't know much about FCP X, so, if it has improved 'audio editing' from FCP 7 that would be a good thing. However, FCP 7 I believe could only edit audio down to 1/100th second increments, and may not be 'good enough' for this sort of work.
Then again if 'close enough is good enough'... then perhaps it is.