There were a few different ways. In the early days, notes detailing enemy troop movements or battery locations were dropped over friendly lines. Spotter aircraft for the artillery would often simply land nearby their assigned battery and deliver their information in person. As the war progressed and communications technology, specifically wireless radio became more reliable and compact, aircraft were fitted with one way wireless sets. These obviously have their shortcomings as well as the person receiving the information can't respond. By 1916, aircraft began appearing sporting two way radio sets. Not all aircraft were fitted with these, only those tasked with missions that required communication such as artillery spotting.