I put in for ACO in Maricopa County many years ago. The process for me included:
Seeing the position available through the county jobs listing
Completing an application
Coming in for a skills test, which included driving the truck and putting the dog in the truck.
That is as far as I got. I just about strangled the dog with the pole, but never managed to get the damn thing in the truck. =P
AZ was in desperate need of COs for many years and had an expedited process for hiring. They are still in desperate need, but aren't hiring right now. Check with your state DOC website for requirements. AZDOC just wanted you breathing at the time, as I recall. You may have needed your HS diploma or GED, but I do not remember.
You're allowed to have a life, you know. I read it in a manual somewhere. - Eugene Matuzak - Timecop