is pretty difficult to use well in ARAM, just as most champions with a fighter tag are not easy to pick up and just do anything useful with in ARAM. Even with solid wind-wall casts you can wind up being a complete non-factor.
You have to know exactly when to pounce on someone's face, when you can slice things and get away with it, and how you're aggression will end before you start.
Otherwise a mistake early can leave you 500 gold off the itemization stats you really need to be a factor, the chance for you to actually make a solid play becomes incredibly low, and low skill rolling the dice to see if lightning strikes will often result in failure.
Yasou's build? That's
+
. Yasou's crit passive is terrible without IE's crit damage bonus, and it's also non-existent without crit chance.
After that? Typical game ends ~33 minutes in with 11,000 gold per player. Chances are you might not even finish you're 3rd major item, as winning that last team-fight and blowing up things like Inhibitors and Turrets give gold you can't spend if you just proceed to win.
The first 3 major items also generally break down into early game, mid game, and late game. You get 1 at each phase - making the first monstrously important, and the 2nd incredibly important. The 3rd should be useful in the components without being fully completed, 4th and 5th are pretty situational.
You can basically justify any pair of boots, so use those as your situational item for early/mid game without deviating from your core. Even Sorcs can be useful with an item like Feral Flare, not that I'd really recommend doing that.
For how to Yasou? Take him mid-lane.
His mobility and projectile shield make for a pretty solid lanning phase against most ranged champions.
His mobility and projectile shield don't make him good against most melee champions.
If you run him into most mages, he'll have an unusually solid lanning phase for a late game champion.
If you run him into a typical top-lane champion: you'll let them have an unusually solid lanning phase.
Which is super-bad if it's a champion like Jax, Nasus, or Irelia balanced around there weak early, strong late. You essentially give them the middle lane you could have had.
I guess it's also bad if it's a champion like Cho'Gath, Volibear, Renekton, or Darius where they just take a dump on your face repeatedly.