I know what it is but don't know HOW to do it.  Does anyone have some detailed instructions?  Pictures would be great.

Installation would totally depend on the product manufacturer you buy.  While all systems tend to FF the BBL. via mounting the bbl. nut. Some also attach to the upper reciever rails.  If you have an idea which products you have in mind that would help.  Take care!

check out:  www.quarterbore.com (I think?) and he has instructions for mounting various KAC forends.  This will give the idea.

Instructions for installing a Knigh't FF RAS.

1.  Remove flash hider.
2.  Knock out the drift pins in the front sight.
3.  Remove front sight.
5.  Remove handgurds and Delta ring, spring etc.
4.  Put upper in vice and use barrel wrench to remove barrel nut.
5.  Install Knights rear cap then install new barrel nut with wrench.
6.  Put the retaining pins in the holes in the new barrel nut at 3 and 9 o'clock.
7.  Screw on the FF Tube.
8.  Tighten the rear cap with a strap wrench.
9.  Reinstall front sight.
10. Reinstall flash hider.