There is two means to file FPLs using SITA. What you can inside the IFPS Zone has already been described. For flights originating or terminating within the IFPZ you may use the AD line within the ICAO FPL and file everything with IFPS. The big advantage is that all amendments that you make to your FPL with IFPS following your filing will automatically communicated to all downline ATC units (refer to the IFPS Handbook for usage of the AD line). If your flight originates or terminates outside the IFPZ you can use a SITA/AFTN gateway address. There is a bunch of these gateways scattered around the globe. However, for flights outside the IFPZ I would strongly recommend to utilize your local AIS, since there is often need to include special addresses (apart from the usual ZQZX addresses), which only become obvious when you read through the AIP.
Cheers
fcit