tsl: I too recently upgraded to a FF handguard, the YHM customizable model. After applying moly grease to protect the threads, I started tightening the YHM barrel nut. A standard AR-15 wrench will do the job. I don't have a torque wrench. I found the aluminum nut from Yankee Hill needed more "jiggling" than the original milspec nut from Colt. It really pays to keep working the nut back and forth, testing its "30 fl-lb limit" until its fully seated and torqued to the next gas tube hole.
One trick that proved useful for me was - when I thought it was finished, I grabbed the end of the barrel and wiggled it. With the upper receiver firmly fixed in your vise, there should be no looseness at all. Should be rock solid. As it turned out, my barrel did wiggle and I spent more time back and forth with the wrench to get the YHM nut all the way down. I got the feeling tiny little pieces of aluminum were being ground off of tiny burrs inside the aluminum threads. In an extreme case, after tightening and loosening the nut for a while, one could remove it entirely, clean out the shavings including those on the upper's threads, reapply moly grease and screw the nut back on again. - CW