WoWDB

Make certain not to kill any Granary Vandals because they are the ones tossing the bags of grain that you need.

I collected the sacks of grain in the area of 52,63 - for those without coordinates, just follow the road which heads south into The Imperial Granary until you are in front of the large building and a lamppost to your left side.

First, clear the area of Tricky Maizers and Kernel Sprites. If you take any damage - including damage which is absorbed - you will lose all stacks of grain.

Next, pick one of the Granary Vandals to catch from. Trying to catch from multiple vandals just doesn't work because you don't run fast enough to reach the landing points of the grain. I preferred the vandal that was throwing grain sacks from the large building (Cornan is inside of it). Remember to right-click the provided quest item so you may  Catch Grain.

Third: To catch the sacks of grain, you must be standing where they land. You can tell where they are going to land by the small, black plumes of smoke on the ground. If you fail to catch a sack, you will suffer damage and lose any stacks you have! If you have your particle density set to Low, you probably will have a harder time seeing these; I recommend turning the setting up to at least Fair, preferably Good. (Game Menu -> System -> Graphics, Particle Density)

You can only carry five (5) sacks of grain at a time, so you must deposit them in the cart that Sungshin Ironpaw left at the granary. The cart is located at 54,60 - on the left side of the road heading north out of The Imperial Granary - and has a large, green arrow bobbing over it. Run onto it to deposit the grain. Note: it appears the depositing the grain clears the  Catch Grain buff on your character, so you may as well mount up and run/fly back into the granary. REMEMBER TO RE-APPLY THE BUFF FROM THE  Grain Catching Sack!

Important tips:

DON'T

• Taking any damage removes the stack you are carrying.
• Mounting also removes all your stack.
• Druids cannot shapeshift or they will lose their stack.
• Shaman cannot Ghost Wolf or they lose their stack.
• "Druids can no longer attempt to catch  Sack of Grain while in flight form." (Source)

DO

• Mages can Blink and use Blazing Speed.
• Rogues can Sprint.
• Warriors can use Heroic Leap.
• Engineers can use Nitro Boost.
• Goblins can use Rocket Jump.
• Enhancement shaman can use Spirit Walk.
• Monks may be able to use Roll and Chi Torpedo (talent which replaces Roll), but I haven't tested it.

Somehow, despite  Grain Catching Sack having a 1 second cooldown, mine likes to periodically go on cooldown for 5+ minutes (I've had it in my bag for awhile; too lazy to do the quest and a couple times has been on cooldown when I've gone to).  Some of the things it shares a cooldown with must have long cooldowns, which makes no sense BUT WHATEVER.