1) Jack up the rear and support on axle stands - You're going to be crawling underneath, so the last thing you need is the car falling on top of you.
2) The fuel filter is located in front of the rear axle, under the rear seat section of floor. If you can't see the filter clearly, now is the time to clean it off - If you can find identifying marks (make and model), double check that its the same as the filter you've bought. If it's looking rusty, now's also the time to cover it in penetrating oil.
3) Look closely at your filter; There's actually 3 bits where you can get a spanner, as there's a union that screws into the fuel filter on each end. You need to undo the car's union from the filter Union; Not the filter union from the filter.
(Top one is the OLD filter, with the exit union unscrewed - You DON'T want to do that! I was supplied the incorrect filter initially!)
4) Undo the exit side first - If you can't see a flow arrow, its the O/S union. Don't be alarmed that fuel will start dripping out - Just have something to catch it in, it's only the contents of the filter. The fuel pump acts as a valve, so you'll not drain your tank!
5) Once drained, undo the other side. You should have something resembling this:
6) You may have to swap in the inlet union over (I did). Also, be careful of the bracket that holds the filter in place - It's quite flimsy and if its been abused, very likely to snap (mine did!). Panic not and hold the filter in place with a big cable tie.