How to NOT Blunder?

Go through ALL of the opponent’s pieces, and think if he can move them
FORWARD and ATTACK you

There are many ways (one being experience and pattern recognition), but the simple one to start with is making sure all of your pieces are defended. If everything is defended, then you probably won't "hang" any pieces because if you lose it, then at least something can recapture. The goal isn't necessarily to have everything defended (although sometimes that is nice), but to be aware of which pieces are and are not defended.

As for not "blundering", even just taking a few seconds BEFORE making your move can help if you use that time to quickly double check to see if you move there, is anything about to be taken? Even this simple "blunder check" can save a lot of potential errors.

Think twice before moving.

Everything you all say is right, but the fact is that in chess a single blunder is enough to ruin everything. And unfortunately ( at least in my small experience ) you are always so "caught" with everything else - plans, calculations, attacks, remaining time - that there is always a moment in the game when you get distracted, your brain short-circuits, and maybe you make a move that you discarded before because you realized how stupid it was... and then you're done.

THe more I play, the more I'm convinced that in chess the concentration factor is essential. I have accepted the idea that I won’t be free on Mr. Blunder too soon.. He will always come to visit me, for a long time...

How do I stop blundering?

At the beginning of each move
1. Check what your opponent has moved, does that move directly threaten something or/and did it open up a discovered attack?

2. Come up with a few candidate moves that look good. Evaluate what to do if you're attacked, getting out of the attack, capturing the attacker, defending sufficiently, counter attacking, etc. If you're not under (direct) threat, you should be developing, taking the center, attacking the opponent's king, attacking his pieces, taking free pawns, pawnstorming, etc.

3. Evaluate your candidate moves. Maybe 1 move obviously weakens your king safety, another loses a piece because you'll get forked, etc.

These steps don't have to take very long, most of them happen automatically when you quickly evaluate a chess position.

Example: The opponent attacks a pawn with a knight.

1. Can't move the pawn away
Can defend the pawn
Can't attack the knight
Can counter attack another piece
Can threaten something of more value.

2. Options to defend pawn with; bishop, knight, other pawn, queen.
Options to counter attack opponent's pieces, his undefended bishop with your rook, etc etc.

Options to threaten something of more value, the opponent's queen, checkmate, a rook.

3. Evaluate which is best, maybe defending with your bishop blocks your development, defending with your queen isn't good because it ties down your whole queen to just 1 pawn. Maybe your checkmating threat is automatically still defended when your opponent takes your pawn, etc etc, you probably know how to evaluate.

Decide to defend with pawn or knight.

These are like the basics of basics of deciding on a move, and every beginner should be able to become better players if they take their time and follow this.
The better you become, the more tactics and positional ideas you can incorporate in this plan and the faster your evaluations become.
This is from the chess.com discord, written by and tomodovodoo posted by R_Doofus
The following is more from R_Doofus:

Root cause analysis for blunders

Blunders are normal, especially in faster time controls than classical chess. What's important is that you analyze the game and moves, and find out why you blundered.
If the analysis of your games concludes that the problem is that you're bad / stupid / can't play right, **that is bad analysis**.

Instead, keep asking *Why?* until you find something that's actionable.
Here is a checklist you can through when you blundered:

1. Did you start a game with a reasonable time control? If you're playing blitz or fast rapid, significant skill is needed to already do that. Start with 15+10 at the very least, some coaches say 90+30.
2. Did you set up a reasonable environment? That means no distractions, no communication, focus on the game even during opponent's moves, sufficient sleep.
3. Did you correctly identify the situation on the board during or directly after your opponent's move? You should play completely differently when you're up a queen vs in an even position, for example.
4. Was your criticality assessment of the position correct?
5. Given your criticality assessment and your clock time, did you take sufficient but reasonable time for the move?
6. Did you consider all reasonable candidate moves for you?
7. Did you correctly allocate time for each candidate move, eliminating most of them quickly?
8. Did you correctly compare and eliminate candidate moves? For example, this involves stopping any calculation as soon as you have one move left.
9. Did you correctly visualize the positions in your calculation?
10. Did you correctly consider all of your opponent's candidate moves? At least captures and checks should be the minimum standard for <1000. For >1000 , at the first move also threats. For >2000, also threats at move 2 and 3.
11. Did you correctly branch out in your calculation tree, eliminating options that were incorrect?
12. Did you calculate deep enough, and abort at reasonable times?
13. Did you spot tactical patterns immediately, and look for them? (This is a necessity to be able to play 10+0 and faster.)
14. Was your evaluation of positions correct, and if not, why did you mis-evaluate a position you visualized?
15. Did you consider your opponent's long-term plans, and was your idea roughly correct? Did you have plans to meet them? (Mostly for longer time controls)
16. Did you double-check or even triple-check your move when appropriate given your criticality assessment and the move? Castling short is probably fine, but a greek gift of Bxh7+ certainly requires some double-checking.

If you're new to it, correct calculation can be overwhelming, which is why you need *plenty* of time in your first games, and this is the reason some coaches recommend 90+30 games at first.
Once the correct calculation process becomes automatic, you can start playing faster games for improvement.

If you can't get to that on your own, analyze with a friend, coach, your opponent, or somebody else.

All this information is found on the chesss.com discord in the #ask-an-expert channel in the pinned FAQ (Frequently Asked Questions). Didn't write any of this, just thought it would be good to share.

If you are playing chess and are human, there is no known way to completely get rid of making blunders at the chess board. But if anyone does find a way please let me know.

Practice a lot of tactical puzzle.

start a daily game, use vacation time

Okay, I should probably rephrase that question. How do I REDUCE the amount of blunders that I make? I know that blunders are unavoidable. For those that are curious, I play rapid 10/0 (the default rapid time setting), and I make a blunder around once every three games.

Mistakes and misses are what I’m more concerned about. Whenever I look at the game review, I have at least 1 mistake and at least 2 misses.

The truth is that people always make blunders/mistakes.

The best way not to blunder is, of course, not to blunder

I used to have a very similar problem, and I stopped blundering after completing a couple of puzzles and courses on this site called: https://chessmood.com/?r=NationalChessBlasters its actually really good, I would check it out!