Unlike in movies, most smart people aren't good in chess.
Sure, playing chess needs intelligence, dedication, and good chess players are smarter than an average person. But it's waaaay exaggerated in movies. I'm a math researcher, and in any movie, my department will be full of chess geniuses. But in reality, only about 10% of them even play chess.
learn how to mate in endgames with a few different combinations of pieces.
Castle early and on the same side of your opponent.
Learn to defend scholars mate.
Focus on piece development early on, get you back rank pieces out (bishops knights)
Fight for the center
When attacking a square, just count how many other pieces are attacking and defending that square and see if you have more than your opponent, this is a great way to quickly analyze an attacks value.
Trade when you have a piece advantage, this is like taking a math question and simplyifing the terms. It greatly simplifies the game and brings it in to the the end game with an advantage.
Learn any one opening system just a few branches that can consistently bring you into tactics (static analysis of the board state) even or with a slight advantage.
These tips can be accomplished in a week and will dominate anyone who 'just knows the rules'
Funny, but really, those things are marginally more effort than learning the rules and are a far cry from the level of effort it takes to actually be considered broadly 'good' at chess.
Learning one opening system can be done in about an hour and most of the tactics advice is just things to think about as you play.