Graeme,
Quite an easy job mate but best done on a ramp.
Soak the nuts and bolts/studs overnight with WD 40 or similar.
Good ratchet, sockets and extensions will be required.
The O2 sims screw into the rear of the cat or de-cat and then the O2 sensor screws into the O2 sim.
The de-cat pipes may have them built in, so you may not require them.
Alex.