It can be aligned by eye if all you want to do is make the rifle function, but a bit more care is needed if you want to do it right. If the gas tube drags on one side, it will wear and eventually pass enough gas that a FTF will result (and it will be merry hell to find your first time). Install the gas block (lightly, so you can remove it easily) and gas tube with the handguards and/or free float tube off, then watch the tube as you slide the bolt carrier group home; if the tube moves, remove the gas block and lightly bend the gas tube in the direction it needs to go. Repeat the process until the gas tube doesn't move when the carrier is inserted. This will minimize wear over the life of the barrel, and even allow you to reuse the gas tube with several barrels; check the diameter of the "mushroom" at the end of the tube - if it's full diameter you can reuse the tube with no problems.