Summary: Based on Eiyu no Kage the challenge is basically Fate Grand Order set in the Naruto Universe why Grand Order specifically because it the best way to explain why Naruto can have multiple servants
How Naruto becomes a Master is entirely up to you, but since i got permission from AzureFlameXIIIGod i suggest using the same way he did, or you can basically have Jiraiya messing around with the Kyuubi's seal.
When Naruto becomes a Master is also up to you but i recommend either during the Chunin Exams Finals before the Sand/Sound or after the Sasuke Retrieval Mission
As for who Naruto has as servants is also up to you but here's who i recommend
Archer: Shiro Emiya
Assassin: Sasakai Kojiro
Lancer: Cu Chulainn
Saber: King Arthur
Ruler: Joan of Arc
But if you don't want too then just go ahead pick whoever you want because Grand Order has a lot to choose from and also so this is a Holy Grail War so there is more Masters than just Naruto like Sasuke for example in AzureFlameXIIIGod story Sasuke just became a Master out of no where with no build up what so ever, if you decide to make Sasuke a Master you can just say he was given command seals from another Master that died or he found a scroll that explains how to summon a servant and if you do here a some servants he can have.
Avenger: Edmond Dantès(Avenger is in his Name)
Lancer: Brynhild(A rival for Arthur)
Saber: Souji Okita(Just because)
Other people I have in mind to become a Master are:
unless you want to use OCs by all means then go ahead
Pairings can be whoever you want but as per standerd norm i'd want to see a Harem with these characters
Fuu(they share the same pain and goals and if she is a Master then the drama gets better)
Tamamo-no-Mae(Naruto holds a Fox demon she is a Fox goddess am i the only one that see's this)
Medusa(GODDAMN Heavens Feel Route)
Joan of Arc(obviously)
and maybe an OC(for fun)
oh and for those who ask Chakara can be a precursor/substitute for Prana
also one last thing like AzureFlameXIIIGod's story there is an after story in which Naruto becomes a Heroic Sprite and participates in the Fifth Holy Grail War as Shiro's Servent why you'll have to find out in the next Chapter/Challenge.
see you later.