Although there are already a couple walk through available, here is one more with lots of pictures.
Make sure you have gasket prep, 1 gallon of coolant, and the two coolant elbows. A breaker bar REALLY helps in tight spaces like this.
Engine block to the idler coolant elbow is GM Part 24503423 and idler to the water pump is 12565082
1.Remove the coolant tank. Remove the serpentine belt by turning the idler pulley counter clockwise.
2. Remove alternator. No need to unhook wires.
2. Remove idler. Pull the idle directly towards the passenger wheel well. Note the direction of the elbows and the end of the lower elbow that goes into the idler.
Coolant will pour all over if you don't drain the radiator beforehand (I didn't).
3. Remove elbows from idler. They might break off in the piping be careful to remove all traces of them so they don't cause problems later. also inspect the engine block to ensure the elbows didn't break off inside there as well.
Broken elbow end stuck in idler:
4. Rather than disconnecting the heater pipes and alternator wiring, I just put them on top of rags off to the side of the engine.
5. I used gasket prep on the ends of the new elbows.
6. Install the elbows facing towards the engine exactly as the old ones were facing.
7. line up the idler with the elbow holes on the engine, push directly towards drivers side wheel well. Once you have the elbows mated, thread all three bolts by hand. Tighten the center most bolt and then the other two.
8. Reinstall alternator and replace serpentine belt.
9. Open the bleed valve located over the thermostat on the drivers side of the engine.
10. Open Radiator cap, fill radiator.
11. Close the radiator cap but leave the bleed valve open and raise the coolant tank as high as you can then tip it towards the hose. This will bleed most of the air out of the system.
12. Once you see coolant spraying out of the bleed valve and no air, close it and reinstall the coolant tank.
13. Refill the coolant tank if needed.