Some guy wrote an extensive guide with just about everything you'd want to know about the AT-SAT. It's stickied in the Applications subforum, but the updated version is over at Stuckmic:
http://www.stuckmic.com/miscellaneous-aviation-information/892-reflections-advice-sat.html
As far as timeline goes... you can just about toss exact times out the window, as they've changed for every public announcement. However, the order hasn't differed. Roughly, the process is:
1. Apply online at ASAP.
2. Wait.
3. Receive approval to take the AT-SAT.
4. Wait.
5. Take the AT-SAT.
6. Wait.
7. Receive your AT-SAT score.
8. Wait.
9. Receive geographic preferences notice, submit them.
10. Wait.
11. Receive notification that you're on a referral list.
12. Wait.
13. Receive invitation for a PEPC (basically a day where all interviews, clearances, etc. are done together). Generally, you should take them up on the PEPC, as doing things separately can take much, much longer.
14. Wait.
15. Go to the PEPC. Usually receive a TOL (tentative offer letter) the same day.
16. Wait.
17. Receive a FOL (final offer letter), which I believe includes your Academy start date.
18. Wait.
19. Go to the Academy. Do not pass Go. Do collect Per Diem, which is, unfortunately, much less than $200.
20. Studystudystudystudystudystudystudy x 1000000.
21. At the end of the Academy, take your PV (performance evaluation). Pass, hopefully.
22. Head out to your assigned facility to complete your training.
That's assuming eveything goes well, of course. I may have left out a few waits.
Corrections welcome, I may have mixed up something... I'm just too awesome to proofread today.