I don't know a guide to this, it basically just comes with experience and knowledge. What you want to do though is analyze your own champ before the game and figure out what they benefit from. A good place to start is looking at their base stats, base damages, and scalings. Use that info to figure out what stats your champ uses well. You can then use either that info or something else to find out what your role on the team is (single target burst, tank, mage, whatever- sometimes you can choose among these in game). Once you get into game, you'll need to do some analysis of their champs. What kind of damage do they do? How much damage do they do? How long does it take them to do it? How tanky are they? Who on their team is dealing damage to you? What items are they building?
At that point you can make pretty informed decisions. Build what you need to do the job you have. So if you're an assassin and you're jumping on them, doing your burst, and they're not dying, you need more damage. If you're jumping on them and dying before you can do your burst, you need more defense. If the malz is ulting you and making you sad, you'd better get qss. There are a million of these situations, so you kind of just have to get a feel for it. As rules of thumb, tanks/fighters benefit really really well from one offensive item and decently from 2, but beyond that it usually doesnt work as well, so you usually want more defense at that point. Mages can go full offense without going full offense, so there's usually like 5 core items on a mage. Last one is just whatever works best that game. Assassins typically want 4-5 offensive items including shoes and 1-2 defensive ones. Adcs want 5 offensive including shoes and 1 defensive. Supports build REALLY situationally, so you have to be able to figure out what your team needs. That is, enemy team does magic damage? Grab locket. All physical damage? Froheart. They're cc-locking your team? Mikael's.
If you play a specific role/champ i can give more specific info
