The celebritiy fp's is a hard thing to explain, but, as others have said, you have to get there early. The more popular/rare the celeb the earlier you have to be there. For example, people lining up for Ray Park got to the park at 6am but if you want to get Ashley Eckstein you could get to the park around 7:30 and still get her autograph.
There's a line for each celeb that forms to the far right, before the turnstiles, there will be a CM holding a sign at the end of each line for each celeb. If you don't see them just ask the person that's last in a line and they'll tell you which celeb they're waiting for.
Around 7/7:30am they start putting the wristbands. They have the same number of wristbands as they have fp's, so if you have a wristband you know for sure you'll get a fp.
around 8/8:30am they start handing out the fp's with your scheduled time to go meet the celeb. Some celebs have 3 times, like 10:15am, 1:30pm, 3:30pm. each session last one hour. The hosts usually have only one or two time slots.
After you have your wristband and fp you can enter the park, then come back to where your celeb is (it's been Echo Lake for the past couple of years) at your fp time, you'll get in line and just wait about 10 minutes to meet them.
They cut your wristband off after you get your autograph and punch a whiole in your fp.
You can only get one celebs fp's for autograph. And they are handed out manually so it doesn't impact your use of fp for attractions.
The celebs will sign anything you bring, pictures, posters, action figures/ backpacks, calendars. etc.
Like someone before said, you cannot bring someone with you to meet the celeb but they can stand outside the area and take a picture of you with said celeb.
If you have kids the CM's will still insist that they wait alone, if they are the only ones who got a fp. But the area is open and you can be right there with them, just not inside the corrals.
I think that's everything that has to do with celebrity autographs.