Summoners are typically trained from an early age for the task that awaits them, and all of them are made aware of the fact that upon their journeys end, they will almost certainly perish: this is one of the major reasons that so few become Summoners, as while most would like nothing more than to see Sin vanquished, few are selfless enough to make this choice. In return for this sacrifice however, Summoners are the only known beings capable of harnessing the power of pyreflies to summon Aeons, beings of extraordinary might whom heed their prayers and descend to their aid when beckoned. The Aeons may only be acquired by travelling to any of the Temples of Yevon scattered throughout Spira and praying in their respective Chamber of the Fayth. If the Aeon answers their prayer, they've acquired it, if for whatever reason it decides not to, it either means that they're not ready or that their values and beliefs are too strongly opposed to that of the Aeon for it to be capable of answering their plea, in which case the Summoner has no other choice but to journey to the next Temple without its aid; occurences like these are very rare however, and most Summoners experience no difficulties on calling upon the aid of most Aeons.
Contrary to what most believe, a Summoner doesn't practise the art of Summoning Aeons during their preparation but are instead taught how to steel their resolve and maintain unshakeable faith and discipline, so that they might serve as pillars to he suffering of the people of Spira. To aid them in this purpose, all Summoners are taught how to harness the energies of aether to perform magic, usually, Summoners are taught the art of white magic to help aid them in easing the pain of the inhabitants of Spira; such as healing the wounded, bringing family members back from the brink of death and other wonders. Beyond this, they're also taught how to perform the Sending, an elaborate ritual that pacifies the spirits of those recently deceased and sends them to the Farplane, a world of spirits that exists on the inside of the planet of Spira and is the place which contains the entire planets life-energy. To become a full-fledged Summoner, the initiate must first travel to a temple of Yevon and present themselves to the clergy to be admitted into their ranks. They would then need to state their decision and undergo the preparation, once the High Priest or High Priestess of that particular temple deems the initiate ready, they must enter the Cloister of Trials and complete it, before reaching the Chamber of the Fayth itself.
Once they've reached the Chamber of the Fayth, the initiate will pray to the Fayth that resides there, and if it answers their prayers - the Fayth will appear before them, and establish a mental connection with the Summoner, the creation of this link is an extremely fatiguing process and its not uncommon for summoners to faint due to the sheer mental pressure. In the case of especially powerful Fayth, they will receive the ability to call upon the Aeon whom serves as the physical representation of the Fayth's Dream.