I know some of you get a giggle out of an easy question like this:)

there are special snap ring pliars, external ones and internal ones. they have a round nose to fit inside the holes on the snap ring and you squeeze to either open it or close it so you can install it.

But where does it go? I told you this would give you a giggle:)

It goes on the barrel extension behind the weld spring and the delta ring.  It's what holds the weld spring against the delta ring.  There is a groove around the barrel extension that the snap ring fits into, but you have to expand the snap ring to get it there using external snap ring pliers which force the ring apart (then you slide it on and release it when it is in the groove).  It can rotate after you get it on so you can align the space between the ends of the snap ring to allow the gas tube to slip through.  Once that snap ring is on, it holds the entire delta ring assembly together, and provides the backing for the spring action of the delta ring which locks in the handguards.  How's that?

Okay, not so good.  I meant BARREL NUT.  Damn, I almost had that one...

While I have snap ring pliers I usually just start one end of the snap ring into the groove on the barrel nut then use a flat bladed screw driver to "nudge" the other end into the groove.

HAHA! I was lost with the reference to barrel extension. I see the groove on the barrel nut and understand it now.
 Mount the barrel on the upper, tighten the barrel nut, put the ring in the groove, install gas tube through the spring and delta ring.
 The one tool I forgot to purchase was the snap ring pliers. So far I've bought a mag block, reciever block, barrel jaws, 25pc punch set, digital calipers, go no-go guages. torque wrench, gas tube install tool, plastic trigger pin punch, and a brass hammer.
 Am I missing anything? Thanks for all the help lads. I shall post pics this week when I get it all put together...

Much easier to install the delta ring, weld spring and retainer onto the barrel nut BEFORE sliding the barrel into the upper. It's pretty awkard to do it the way your posted.