The 4 pillars of a harmonious progression.

While it is important to practice with exercises and practice games (the rule : more slow games than fast games and especially playing against opponents stronger than you), it is also essential to learn. It will not only increase your knowledge (invest in the series written by Artur Yusupov), but also correct your faults thanks to the analysis.

These are only guidelines to help you. Below 1500-1600 elo, the main bottlenecks are related to poor thinking process and shortcomings in the calculation. Chess literature, videos, coaches will allow you to complete the areas briefly discussed.

1. THOUGHT PROCESS
It can be summed up in three steps:

• Evaluate the dangerousness of your opponent's move
• Pick the best of your two candidate moves.
• Make sure the move doesn't degrade your position both tactically and strategically.

During your reflection, and for at least 1.5 moves, you must first think, and in that order, of the check possibilities, then of capture and finally of threat. As much for your moves as for the opponent's responses. This reflection should be carried out until a position of equilibrium is obtained.
Overall, you will therefore have to establish priorities during a game:

1. Respond to the potential threat
2. Evaluate tactical opportunities
3. Ensuring the safety of the king
4. Develop and activate your pieces
5. Think strategy

2. TACTICS

Get comfortable with basic tactics first by studying them one at a time.
Then work with general tactical exercises. Various sites make such exercises available to you.
Tactical exercises : among a stock of several hundred exercises, work several dozen daily while trying to understand why you have not found the solution. You will repeat them over and over again until you can solve them in a few seconds by dint of seeing them again. Then start over with another, different or more difficult stock.
Calculation exercise : reflect on a mid-game position from a game of great players, analyzing all the possible options and evaluating the final position of the analysis of each line (who has the advantage?). The minimum duration of this exercise: 15 minutes
Take advantage of these exercises to establish an assessment of the most frequent errors.

3. ANALYSIS

Each of your long games will need to be analyzed. The use of an analysis engine will only take place in the very last step, after you have thought through all the variations for yourself.
The analysis has several purposes:

• Find critical moments in the game.
• Look for the reasons for your own mistakes: not just what went wrong, but why it went wrong.
• Seeking new possibilities: what moves have you missed in your analysis? What ideas might you have considered?
• Think about the opening.

You will have to formulate in writing the reasons for your hesitations and fears during the game. After each analysis, you should have learned at least one thing. For example: "Finally a game where I felt comfortable", or "My king is rarely safe at the end of my opening", or "I don't know how to lead a pawn ending"
Game after game, you will be able to take stock of your main weaknesses that will need to be corrected.

4. TRAINING PROGRAMM

This program (you can use other ideas) will be based on 2 to 4 sessions, at least 1 hour each, which you will repeat, day after day, session after session. Only start the next session if you have finished the current session (the same session can therefore be carried out over several days, even if it would be advisable to be able to do one per day: adapt your work volume accordingly).
The program should include, in varying proportions depending on your weaknesses and level : the study of openings (reduced to a minimum for a 1200-1500 elo), midgames, and endings. And for each of these game phases: theory, practice (exercises) and review. It will also be necessary to add exercises of tactics and calculation as well as parts of training games which will be analyzed.
Plan at least 30% of your planning for tactics and calculation, and 40% for practice games and their analyzes.
On the remaining 30%, focus on the ending. And depending on the time available: work on openings and the middle game (strategy, reading commented games).

Finally, change your program every 2-3 months according to your improvements.

Nothing miraculous, I simply tried to summarize and synthesize what I read here and there.
You do not agree ? I forgot something ? Tell me !!