How did they locate target and relay that information back to the artillery battery?
There were a number of ways that the artillery identified targets, these included though were not limited to:
Let's look at Aerial Recon and photography first. The British started using aircraft to identify artillery targets in 1914. There role was to locate targets, mark it on a map and communicate this to the artillery battery. They would then observe the fall of shot and advise corrections based on that observation. According to Bidwell and Graham, the aircraft would also return periodically to ensure the artillery fire had not drifted away from its original target. As the war progressed, it quickly became apparent that the priority of the various air forces was the location and identification of enemy artillery batteries and until the introduction of Flash and Sound Ranging, this was the main means of locating enemy batteries for Counter-Battery Operations. The communication of targets from the aircraft to the batteries evolved throughout the war. Pre-war experiments with aircraft spotters tried smoke signals and wing flapping but found the best method to be to simply land beside the batteries and hand them a note or a map with the location on it. That was until the introduction of wireless. While the French and Germans had made decent progress in this regard, the British had no wireless equipped aircraft when war broke out. By 1915, wireless flights were being incorporated into the Royal Flying Corps and by mid 1916, aerial observers could communicate with any battery they needed to.
Forward Observation Officers. These men, as their title suggests, were stationed at the front and were responsible for observing the fall of friendly shells in order to determine if they were falling on target or required correction. Corrections were advised through the use of field telephones though these often got disconnected when enemy artillery fire severed the line running to the rear. These men were not really responsible for locating targets as their positions more often than not limited what they could actually see but remained and still remain an integral part of the artillery.
This brings us to queen of the First World War artillery techniques, Sound Ranging. In 1915 the British began working on a means of locating German artillery batteries by measuring the time it took for the sound of a gun firing to reach six microphones placed along the front. By measuring the different times, the location of the gun could be determined through the use of trigonometry to within 25 yards. Unfortunately, the men responsible for making this work were stuck using French microphones that were ill suited to the job. In 1916, they received the 'Tucker microphone' a device far better suited to detecting to low boom of a German gun. When refined, the use of Sound Ranging could accurately determine the muzzle velocity and calibre of the detected gun. The first major test of Sound Ranging came at Arras in 1917. The Battle of Vimy Ridge saw the accurate detection of 86 percent of German guns, most of which were neutralised by British and Canadian artillery.
Edit: Sources
Firepower: The British Army Weapons & Theories of War 1904 - 1945 by Shelford Bidwell and Dominick Graham
Battle Tactics of the Western Front: The British Army's Art of Attack 1916 - 18 by Paddy Griffith
Artillery in the Great War by Paul Strong and Sanders Marble