This may vary on your vehicle as this was done on my 08 impala with the 3900, it should be the same or similar as your 06-07 monte carlo

I replaced the lower one, locally best price I found was $66 but Rockauto.com had AcDelco for $34 so I went that route, this could be used for any of the 3 idler pulleys I could probably get a cheaper one but if I can replace it with AcDelco I will
2015-11-08%2020.32.03_zpsuxbtlsnq.png

20151105_183614_zpsjgakr9jj.jpg

Tools needed
3/8 ratchet
10mm wrench
13mm socket
Piece of pipe (not necessary but helps)

Step1
Pop the hood
20151107_145945_zpshczsb1pn.jpg
20151107_145956_zpslzjpb324.jpg
IMG_20151107_163318_zpsxx1yzxfm.jpg

Step 2
Remove resivour and set aside
2 10mm nuts
20151107_150050_zpsk9wdmdez.jpg
20151107_150118_zpsvbfm65zf.jpg

Step 3
Lossen up your belt and pull it off the alt
Use your 3/8 ratchet and inset it into the tension or to do so
20151107_150347_zpswmzvbwlm.jpg
I use a pipe for a little more leverage
20151108_133100_zpsq1ushuwa.jpg

20151108_133134_zpshzstvh7v.jpg

Step4
Locate the one you will be replacing
Upper smooth one
20151107_150127_zps6fo2jhfo.jpg
Upper ribbed one
20151107_150151_zps4wdhczbq.jpg
Lower smooth (the one I replaced)
20151107_150145_zpskmo8fcaj.jpg

Step 5
unbolt old pully
Mine was a 15mm
20151107_150729_zpsbhx0lmxg.jpg
20151107_151006_zpshh7cpulx.jpg

Step 6 install new pully
20151107_150550_zpsdevbbboi.jpg
20151107_151049_zpskabiexfd.jpg

Step 7
Reinstall belt
(See step 3 and reverse)

Step 8 reinstall coolant resivour
(see Step 2 and reverse)

Step 9 shut the hood

Step 10 pat yourself on the back for a job well done