Here's what you do:
1) Get a set of action blocks and a big vise.
2) Remove handguards, gas block (or front sight) and muzzle break (if it won't fit the hole in the FIRSH, most won't fit.)
3) Lock the action in to the vise. Remove the barrel nut, using the special nut wrench. Note: if it won't come off, come back and post an amusing thread about all the stuff you tried before ruining your upper receiver. It should come right off... but there are a few amusing threads floating around in "build it yourself."
4) Screw in the FIRSH nut. Index the handguard, and tighten the underneath screw.
If you have all the tools around, you probably would not be posting here!
Alternate Plan A- take the rail and $30 to your gunsmith.
Alternate Plan B- buy a new upper with a FIRSH on it! Keep both uppers, then buy a lower to match your new upper.
Pete
ps- I'm also looking at the Badger STABILIZER handguard... similar. I don't know much about it yet.
Check out the AR-15 thread on uppers and such. They have a great review of the handguard posted at the top of the thread/