We did Fantasmic on Tuesday, March 25th. You go to Guest Services earlier in the day (I went on the way back to the hotel for our mid-day break, and there was no line) to pick up your tickets. Guest Services are on the left-hand side of the
Disneyland gates as you are facing the turnstiles.
You check in at the riverside area where they are setting up. They pull out a podium right around 6:15-6:30. It is next to the churro cart in front of Haunted Mansion. When we were there, the line started at 6:30 when they pulled out the podium, but they did not start checking people in for the 9 p.m. show, until the stroke of 7 p.m. When you check in, they let you pick your seats, and it is first come, first served. They wrote our seat numbers on our tickets. Another guy with a clipboard was checking in guest for the 10:30 show and would periodically shout, "Anybody checking in for 10:30? Check in with me!"
Then you wander off and shop or ride a ride until about 8:30 p.m. You go back to where you checked in. At that point there is another line of people waiting to get seated. They take your tickets, look at the seat number on them, and escort you to your seats. There are cast members who are assigned to several rows each of guests, and they will quickly come over and bring you a LARGE dessert box (4 mini-desserts -we had a key lime tart, a chocolate decadence brownie, mini-cheesecake, and a big fat macaroon--, a HUGE choc. strawberry, 6 little mini-cheese packs, 3 packs water crackers, 2 truffles, fresh seedless red grapes) and ask what you want to drink. They'll check if you want refills just before the show starts. Then after the show they will ask again, before the fireworks start.
It was WELL worth it to us to be able to stroll up and have that kind of seats and service for the early show. No way would my little ones have been able to hold out for 10:30. If it were just adults, yeah, I'd do the late show and no special seating.