Duff-Man wrote:I didn't build my upper so I didn't answer that part. But I have taken it appart and put it back together for various reasons, new handguards, boredom.....
I think I could do it all now, but I've watched so many videos on it and still do from time to time.
Ya may as well go ahead and tick the upper, too!
... only part worth mentioning is the bbl-nut (
presuming ya had to replace it, for at least one of the handguards? 
) ... and of course, the gas-block pins, if ya don't otherwise use a set-screw or clamp-on model?!
Just to clarify for those that ain't done it ... there's two types of "
assembled" uppers ... there's the "
complete" upper (
which is ready to shoot ... once ya slap it onto the lower) ... and then there's the "
assembled upper receiver" (
which already has the ejection-port cover installed, and the forward-assist ... if so equipped) ... the latter, otherwise being a "
stripped upper receiver" (
at which, ya got a little fumbling with a microscopic e-clip for the ejection-port cover, and a roll-pin for the forward-assist assembly).
Presuming you're starting with an "
assembled upper receiver", and you're gonna do a lo-pro set-screw gas-block and a free-float handguard, ya simply set the bbl into the receiver, slide on the bbl-nut (
comes with most any free-float handguard) and screw it down onto the receiver (
this "clamps" the bbl in place) ... the bbl-nut will have little holes all the way around ... ya need to line one of 'em up, with the gas-tube hole in the upper receiver (
without loosening the bbl-nut ... IOW, ya gotta go tighter, if at all) ... the gas-tube is usually pinned into the gas-block (
no big deal, with a proper roll-pin punch) ... slide the gas-block (
preferably with the gas-tube already pinned in) onto the bbl ... it should slide on, until it meets a ridge in the bbl ... there's also a hole in the bbl (
gas port) ... this should line up with a corresponding hole in the gas-block (
very near where ya pinned the tube) ... tighten down the set-screws on the gas-block (
there may also be dents in the bbl lining up with these screws) ... the handguard then simply threads onto the bbl-nut (
if there's a jam-nut, that should go on first, then the handguard until it's close to the jam-nut, "clock" the handguard as ya like, then turn the jam-nut against it, to lock it down ... this'll require a spanner-wrench of sorts) ... aside from the muzzle device, the BCG and latch slide in/out (
as they otherwise would for regular cleaning, anyhow) ... done!
Ya may also wanna use some anti-seizing compound on the bbl-nut/upper-receiver ... and some loc-tite on the gas-block set-screws ... and a crush-washer with a muzzle-device!
An upper-receiver block can also be used to hold the upper in a vise, whilst ya do some of that stuff!
ETA: once ya "
built" your first upper, ya'll probly never buy another "
complete" one, again! ... that'd take all the fun outta it!
(
... won't happen, again! 
