For the rears, Remove the rear seat (Horizontal part of seat first, then the vertical part), then remove the parcel shelf.
In the front, you need to remove the door trims. There will be a couple of screws you need to find, then its just a matter of pulling the trim away from the door to undo all of the clips that hold it on, then sliding it upwards, then undoing any electrical connectors for electric windows etc.
That's all you need to do to get to the speakers.
Very simple, just get your tools out and give it a try.
Dan.