first, Learn the London as white and modern defence with e3 as black. Or take a pick at an opening for both sides. Do not play anything else. Do not try to learn a hundred moves in a hundred openings, stick to one. Just one.

Actually study the opening. (just keep on playing it in the meantime.) Take advantage of common mistakes.

Second, When behind: keep playing the entire game. Do not quit. Do not give up. Do not surrender. Your opponents most likely do not know any endgame tactics.

When ahead: Get rid of everything, exchange everywhere, every opportunity to trade, take it. Get to the endgame asap.

Third: Puzzle. Puzzle. Puzzle.

Fourth: Watch high level commentary on high level games a lot.

Fifth. Analyse yourself. What did you miss? Where was the game turned around?