From: Bratislava, Slovakia
Each mission was tailored to fit the requirements and capabilities of the airplanes that were on hand for such a mission. So you would hardly find something like "SOP" for the ground support.
Altitudes varied depending on the target area, nature of the target, accuracy required, bomb load carried/fuses, expected outcome, etc.
You need different type of planes/formations/bombs on a dump/camp area, completely different when trying to silence enemy artillery battery, completely different when trying to suppress a pesky gun on the hillside, infantry in the caves, enemy entrenched on top of the hill, or even if you want to clear area of foliage!
Ground troops usually send a request a day before, with target data - and air force experts then looked at the planes at disposal, bomb load (size, cluster, daisy-cutters/fragmentations, napalm, depth charges, fuse settings), protection, communication, target marking, approaches, and all other stuff and informed the ground troops about what they came up with.
On call (or CAB-RANK system):
Air force sent planes in groups on scheduled times to relieve another flight with some secondary targets. Say 3xB-25D1 over the beachhead between 0900-1100, another flight following. When called on by radio they would target whatever ground troops requested - but usually carried a GP bombs, and added strafing. Or 4xP-40s every half an hour or so. When not called upon in their assigned time, they would target something more distant from the frontline.
Try to read detailed account on Operation Brewer (Admiralty Islands campaign) - it had everything - heavy bombers, mediums, fighter bombers and most of the missions would fall into CAS classification:
Chapter 11 - The Admiralties