I know that in the later stages of the war, aircraft were used to find targets for the artillery, but I want to know exactly how they did it. Did the pilot have a map with a grid, and radioed the battery to fire on a particular square?
Did the pilot have a map with a grid
Yep; they'd typically carry a map with them, then identify the target's location on a particular square, then they'd either radio it in using the wireless, or use a weighted container to drop a message onto the battery.