Proper way to wire a sub panel to an attached garage?
When wires get this large, using copper is just throwing money away.  2AWG aluminum is a very appropriate choice for this application.  Whatever ooga-booga you may have heard about aluminum, never applied to the large conductors like this, and doesn't apply to the new AA-8000 alloys.

How about EMT steel conduit?  Use that, and you don't need to pull a ground wire.  The EMT is the ground.  (Not only that, it's ground for any other circuit that needs to be grounded back to that panel, in case you are retrofitting grounds in an older home). 

If possible, I would use 2" conduit.  I know the tables say 1.5" is adequate, but this assumes you have an electrician's truck chock full of pulling tools and I'm guessing you don't.  The more oversize you go, the easier the pull, and the less likely you'll need to hire the guy with the truck.  And guess what, he won't work with you unless you hire him for the whole job.  I managed to hire one just for the pull, but he was a friend of my family, and even he needed to delay the job a day so he could buy another tool he didn't have.  Anything you can do to ease the pull is a good idea. 

Watch the bends.  Every bend makes the pull much harder.  When I lay conduit, I allow myself only one 90 degree bend between accessible points (Code allows 4, but you'd need a power winch and a bunch of lube to pull 3x#2AL around 4 of them! Yikes!)  Whenever you use a junction box or conduit body at a 90 degree bend, that replaces the bend with an accessible point!  Quite a bargain!  It also makes the turn sharper and neater.   If it were me, on a big one like this, I'd put an access point at every single 90... which would make the pull easy, and could make it workable with smaller pipe.  I still wouldn't go smaller than 1-1/2". 

Food for thought (i.e. where to use a junction box instead of a conduit body), you can have up to 4 circuits in a single conduit.  (literally, 9 conductors, but grounds don't count, and neither do neutrals on split-phase 120/240 circuits and MWBCs, so you've only used 2 of 9.)