Have to take the lower half of the back seat out.
At the front of the seat down towards the floor there's two 10mm bolts (one either side in the footwells). Take them out, and pull the cushion up at the front and towards the front of the car.
The bolts for the the buckles (17mm) are near the centreline and the bolts for the lap belts (17mm) are towards the doors.
From memory the configuration of the washers, spacers, etc. Is -
Bolt head
Tie-down for belt
Thin wavy washer (from memory)
A spacer - one end will be narrow for the belts tie-down to slip over
Spring washer
And either a paper or plastic "washer" that screws on to the bolt. It stops everything from falling apart.
Getting that cushion back in can be painful at times. If the bolt holes are hard to line up, try sitting on the cushion above the bolts.
Hope it helps.