Have a look at the video tuturials for remova of lowers and air spring service on the tech section of the Rockshox website,then refer to the Reba manual.
I did mine recently.In essence:
1) Remove forks from bike
2) Release air in positive and negative chamber
3) Remove lowers
4) Remove air spring and remove 1 spacer from air spring shaft.
5)Replace oil and reassemble.
Taking care and following the instructions it took me approx 1-1 1/2 hours.Could probably do it in half that time,now
If you are mechanically mided and take your time,it is surprisingly easy.
You will need some 5 and 15wt fork oil,(get it from crc)a selection of sockets,eg 10,15,24mm ,a syringe to put the fluid in,circlip pliers,and a large rubber faced mallet.