Guides and stuff can factor in a lot of things. They can recommend items for matchups, how skilled you are, and do a great number of things to start you out on the right foot.
The one thing they can't take into consideration is your play style. Many champions have multiple viable build paths you can take which fully depend on how you want to play the champion. The only way to properly account for this is to learn it yourself, or have someone spend a whole lot of time watching you play. I personally recommend the former.
Even if you don't use them try setting up some custom item pages for yourself. Put a bunch of items into categories like Magic Resist, Armor, Health, Damage, and anything else you find fitting. Place anything in there you think might be a good item, and as you play test them out. If you don't like how certain items worked remove them from the list, and keep going until you have a fairly reliable set of items that you like using. Once that's done just go with whatever feels right for the situation. You're training your intuition here, not making a spreadsheet.
