Use the 40-40-20 Rule: Study 40% middlegame, 40% endgame, and only 20% opening.
Use "Prophylaxis": Ask, "What does my opponent want to do?" and prevent it if it hurts your positions
Speaking of positions, Use CCTO: Before every move, check for these three things in order: Check, Capture, Threat.
Analyze Your Games: Always review your games to find moves that hurt your position.
Take your TIME: ALWAYS take your time, especially if it's a bad position. You'll probably soon realise that in a specific POSITION, YOU 🫵 could make a better move.
Active King: In the endgame, bring your king to the center to assist in defences or even attacks.
Do not do ONLY puzzles. Your profile picture probably looks like: Rapid: 1000 Puzzles: 2500 (mine looks like this too 😁). Puzzles are helpful, but chess games can and will have more information to help you improve.