This gets easier as you become more familiar with all the items and their interactions.

I maintain my own builds for all the heroes I play.

When I pick up a new hero I generally play a few games completely improvising, so I get a feel for what does and doesn't click for me with that hero, so I can cover my weaknesses and build to my strenghts.

As an example, when I started off with Mina, I went for fire rate to make getting love bites easier. Eventually I noticed my feel for her gun was good enough that I no longer needed it.

I plan out three tiers of items to try to buy every game, and then I have an "optional" category of a bunch of stuff that might be contextually useful, or fun to try.

If I find something fun or effective, I move it to the main categories.

I have a general idea in mind and that changes during the laning phase. What do they have, what do we need? The counter build is the side quest and getting my build online is the main quest.

Usually I'm braindead and follow build L -> R but I learned to try to jump item to keep more money. I don't feel confortable yet trying to build my own build but I feel I'd learn a lot by trying to build one