What's your thought process for deciding to add/remove card(s) from your deck?

Serious question. As the title suggests what goes through your mind when deciding what to put in or take out of your deck? I'm having trouble with my deck building so I want to know how you guys decide the best way to alter your deck. A lot of times people put cards in their deck because they saw it on some video online and there's nothing wrong with that if you know why those cards were added in the first place. I want to know how you as a deck builder decide what's useful in your deck and what's not.