Its pretty strait fwd.
use a punch to knock out the role pin holding the gas tube in, use a punch on the taper pins for the front sights, pull out the gas tube, pound off the front sight block. Unscrew the barrel nut w/ the correct wrench.
I have a two piece DPMS vented free float tube. Remove the tube from the nut, apply some moly greese to the threads, thread on the nut and torque to around 35ft-lbf? Align the holes in the nut to the hole in the reciever for the gas tube. I have a chopped gas block (dpms) w/ a picatinny rail on the top of it that attaches w/ set screws. Line that up vertically as best as possible and tighten the set screws, install the gas tube and role pin. Screw on the FF tube to the threaded barrel nut.
I hate to spend $40 on a action block and $30 on the wrench for something im only planning on using once
_Notch_ got some good reading if your intrested about learning how to do it yourself
http://www.ar15.com/forums/topic.html?b=3&f=4&t=226782