Check their alignment side to side...as in, stand on the side of the car and make sure they're all parallel.
I haven't done alt pulley, so just imagine it's four bolts or whatever to some decent torque!
The idler pulley is also straightforward...counterclockwise gets it off. note the alignment notch and make sure it goes in flat.
If alt is out of alignment, there's a pesky brass threaded bushing holding it in place that seems to get pulled out from either overtightening pulley, or aligning it improperly...as I'd done and missed for about 4 months...I got that long bolt into front of alt, but not in the tab for it in back.
Make sure your belts are the right length, of course, and if you got any fluid on them, clean them with water.
Say an appropriate prayer facing Munich, and good luck!