Visit seatfillers.com and fill out an application. A resume and photo will need to be submitted. The photo does not have to be professionally taken but should be clear and recent.
Check your email daily once you are accepted as a seat filler. Seat fillers will notify you by email if you are invited to be a filler for a particular event. You usually only get a few days notice. Only accept the assignment if you are sure you can attend.
Read all instructions carefully. You will be instructed on how to dress as the event may be formal or semi-formal. Information such as where to go at the venue and arrival time will be included. Keep in mind many major Hollywood events are televised and a neat and appropriate appearance are a must.
Understand your role. Once the event starts you may be instructed to fill a seat for a celebrity or other invited guest until they arrive. Once the celebrity arrives, you will be ushered to fill seats as needed, such as when a celebrity steps away to use the restroom. Keep in mind the point is for all seats to be filled for the camera. You will likely be moving around a lot.