How replace and put back on idler pulley n serpintine belt
Use socket wrench to turn serpentine belt counterclockwise at tensioner to get some slack.
Pull serpentine belt off topside of idler pulley and put it underneath idler pulley, and actually try to tuck it up inbetween engine block and idler pulley. Then close your engine hood.
Then crank engine a few seconds and the belt should flip that idler pulley right off.
Dont even need to start the engine. You can tell when the job is done by hearing a big thud, and seeing the dent pushing out on your engine hood. Turn your key off immediately after hearing the thud, or if the engine starts. You just want to crank it a second or two. Easier than using a puller.
If the idler pulley is really shot to bits, it may come off in pieces; if not, there should be one center bolt and a largish washer. Don't lose the washer or bolt since your new part does not come with those (mine didn't!). Replace everything in the same order. Make sure you follow the torque specifications for the bolt when replacing it.
For the dent on your hood, just whack it back down with a hammer, then spray it with white Rust-Oleum spray paint, or the color that matches your car color.
Hope these help.