When you plug the barrel into the upper receiver, which should be chucked up in your vise in an action block WITH the insert in place of the bolt carrier, there is a little slot at the top on the receiver. There is an indexing pin on the upper side of the barrel that engages that slot.

Now, put some moly grease (ordinary wheel bearing grease) on the threads of the receiver and snug it up by hand. Do this several times to spread the grease. The purpose of the grease is to help give a true torque reading, not read the friction of the nut and receiver.

To smooth the threads, using the barrel wrench and torque wrench, tighten the barrel nut to 30 ft-lb. This AIN'T rocket surgery, as my wife says. Back off, and tighten to 30 a second time.

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edited to add:

Back off again, then torque up the third and final time.

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Before we tighten the last time, let me explain... the torque to tighten is simply an amount that will keep it from backing off by itself, and make sure everything is fully in contact, solid barrel to receiver mount. And also that the nut is not overtorqued, stripping the thread, which happens over about 80 ft-lb. There is no magical torque reading that will make the difference in a good running rifle and not so good. It has NOTHING to do with headspace.

OK, now, this time tighten to 31 ft-lb. Look at the notch nearest the hole where the gas tube enters the upper. Will the gas tube pass through without touching the barrel nut? You can use a drill bit shank, or the gas tube itself. Stick it in through the notch and into the hole. Will it swing from side to side equally? If so, you are done.

Do not back off, you were at 31 ft-lbs, the minimum, SO, budge it a little more. Now check. You want the gas tube to go through and not touch the nut on either side. The gas tube needs to be able to "float" slightly, and flex, to smoothly engage the gas key when the bolt carrier moves.

Gas tube clearance should be easily obtained under the 80 ft-lb mark, and usually is done under 60.

That is it, done.