TIghten, loosen, tighten, loosen, tighten, loosen, tighten, loosen - you'll hit it. If the barrel nut and the threads on the upper receiver are NOT lubed, you'l just make it harder on yourself.
The torque spec is 30~80lb/ft. Somewhere between 30 and 80, you'll hit it. Don't try it with dry threads. You'll gall the poop out of everything.