Keep in mind that the Mega arms billet uppers require a different receiver block than the forged uppers do. All it takes is the correct receiver block, an armorer's wrench, a torque wrench if you want to be really technical, and some blue loc-tite if you want to enhance accuracy. The loc-tite is often applied between the barrel and receiver extension, and between the gas block and the barrel (apply it so it doesn't get into the gas system though).
The Loc-tite isn't a requirement, it is one of the little things that some folks use to get that little bit extra accuracy. If you have a forged rather than billet receiver, some people also want to true up the portion of the upper receiver where the barrel extension meets the upper receiver. I don't know about how well Mega uppers are made as I have never built on one, but many other uppers benefit from truing up the face of the upper receiver.