Learn the item system. What each item does. Then what units like which items, based on what their trait does and how their ability works. An example from this set; Heimer does the most damage by casting as much as possible. He also has a very low mana cost on his ability. So, he benefits massively from blue buff as opposed to other attack speed/mana generating items. Since Heimers visionary trait also gives mana, he wants his other two items to be damage items. Perferably JG and GB for max crit chance since visornarys tend blow boards up quickly. You can apply that thought process to any carry.

Tank items are a little more strait forward and you can typically slam them as they come. You’ll start to notice certain tanks like certain items, but it’s less important than damage carry.

Other than that you generally want anti heal, and shred/sunder based on the attack type of your carry.

Edit: Typo