Not Sure What To Build? Follow These Three Simple Tricks:
- If your champion's ability(s) has % based numbers, try to build items and/or use the ability in a way so you use it with the biggest numbers possible.
- If your champion's ability(s) has flat based numbers, try to build items and/or use the ability in a way so you use it with the smallest numbers possible.
- If your champion ability has a stat that is multiplied with other stats, try to synergize with those other stats.
Okay what does this mean in english?
Let's take an example like Skarner. From his passive to his ultimate, Skarner has: Flat Movement Speed Flat Bonus Attack Speed % Mana Flat CDR % Maximum Health % Movement Speed
Meaning he wants to build a mix of movement speed, items that synergize with attacking a lot consistently, mana, %CDR, and health.
And as it turns out to be, if you look up Skarner's highest win rate build you have:

Items that perfectly match up with what I described.
Cinderhulk synergizes with Skarner wanting health, and the move speed that Chilling Smite provides contributes to the MS mix that Skarner likes. Merc treads allows Skarner to function over time as much as possible despite CC. Trinity Force grants move speed, mana, and health along with synergizing with the CDR effects caused by Skarner's Q. Dead Man's Plate is another element of movement speed and health. Sterak's Gage contributes to health and the previously built Trinity Force. Finally, the Frozen Heart rounds off Skarner's CDR and mana wants.
It's actually a really simple trio of tricks, but it could go a long way to helping people learn how to build.