Pick a date and showtime. Use the Disneyland website to see a calendar of hours and showtimes for Fantasmic. Use this to plan that day's activities.
Show up very early. The viewing area for Fantasmic is along the waterfront in New Orleans Square, facing Tom Sawyer's Island. You would be surprised, but people will start marking out spots and assigning family members to hold them as much as 2-3 hours ahead of time. The cast members (employees) understand what's going on and won't chase you away unless you're impeding foot traffic or affecting safety.
Stake out your spot. A thin blanket is nice to mark your territory and protect your backside from the pavement. You will be sitting on the ground if you choose this method. Is Fantasmic worth it? Ask the people waiting around you. Chances are, this is not the first time they've done this. Get as close as you can to the iron railing for the best views, but be aware that you may get splashed. Park a responsible family member there and then go have fun, taking turns waiting.
Consider the dessert option. For $59 plus tax, you will get a prime viewing location, in a real chair, in a special roped-off area. Reserved seating for Fantasmic is one of Disneyland's open secrets. Besides a great, comfortable seat, you get a box of Disney dessert treats to enjoy during Fantasmic, and a beverage served by a cast member. You will need to reserve this as much as 30 days in advance. The phone number is (714) 781-4400. You cannot do this online. When you arrive at Disneyland, pick up your passes from Guest Services, either at Town Hall in Disneyland or outside in the turnstile area.
Enjoy the show!