MLB The Show 21

From the main menu, select the March to October game mode.
Select one of the teams from the Favorites section (left side of the screen); I personally picked the Los Angeles Dodgers since they have a great team all around for pitching and hitting.
For the difficulty, select either Beginner or Rookie. (I don't rememeber which one I picked, but it was between these 2)

Throughout the season (162 games), there will be multiple "Key Moments" where it will give you a challenge to do in-game such as, finishing a no-hitter, perform well with a certain player, win a game, etc. If you complete or fail the Key Moment; 3 things will happen. If you complete it successfully, your player will receive a boost for the rest of the season, or you'll gain momentum for your team which will help you win games that you're not playing. If you fail a Key Moment, you'll receive a cold streak which will make games that you aren't playing a little harder to win (they usually give you a easier Key Moment where you can get your momentum back up after a few games!).

Since I picked the Los Angeles Dodgers, I had little to no trouble making the playoffs because I came out on top of my division with 102 wins! I still lost multiple games that I played just because I struggle with the timing for hitting.

NOTE: If your team did not do good enough to make the playoffs, you'll not get this achievement and will have to restart the March to October mode from the start again, because you'll need to make the playoffs for Division Series achievement and the World Series achievement also

Once you're in the playoffs (after the regular season of 162 games), the first series will be a Best of 5 series. Each game will be a Key Moment where you just need to win the game/s. After you win 3 out of the 5 games to advance, you'll get the achievement a little after you see the "success" from the Key Moment or once you're in the playoff bracket.
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Once you're in either the NLCS (National League Championship Series) or the ALCS (American League Championship Series), you will have to face another team; but this time will be a Best of 7 series(win 4 of the games). I found this series more difficult than the Division Series. I won the first game, then lost the next 3 games. I somehow came back from a 3-1 deficit and won the Series 4-3!
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Now we're in the World Series! Same thing as the previous series, this series will be a Best out of 7 (win 4 of the games). For me, this series was easier than the NLCS; I was up more and all I had to do was either score a run or 2, or keep the lead.

Nothing more to say, other than, if you're struggling with hitting or pitching, I would recommend going to the practice mode at the bottom of the Main menu.

Once you win the World Series, the achievement will pop once the game finishes and brings you back to the schedule screen!

If you vote negative or if there are any things that I missed or I should add, leave them in the comments, and I'll update the guide!

First, set the difficulty to beginner. Next, I picked the Dodgers thinking they gave me the best chance of getting this achievement. Anyway, you mostly start in the late innings (6th-9th) unless you're player locked; then you play the entire game as that player trying to give him a permanent boost for the season. NOTE: Only sometimes when he's on the field and also every at bat. If you're the pitcher or catcher (I was never the catcher), it's every pitch when you're on the field. When you need to give a boost to a fielder, 1 hit works (great for the minor league call up achievement). For a pitcher, I was getting boosts for double digit strikeouts or shutouts. Dustin May was awesome (he was one of my callups). I used him to shut down the Yankees.

Win to build momentum. Complete challenges to build momentum. Win a certain way (blowout, walk off, shut out, extra inning walk off, etc) to build even more momentum. This brings you wins when your games are being simmed.

Don't worry too much if you lose during the regular season, since there are 162 games to play and losses don't mean much if you're steadily winning. Don't exit the game via the guide. If you do that too often, you WILL lose rewards.

The game tends to punish you for doing too well. Example: I had an endless winning streak and the game had me continue the streak by being down by 1 in the 9th with two outs, (IIRC) two on.

Later during the playoffs (you have to play each game but only the later innings), after winning a couple games, the game would put me in a situation where I'm down a run or two in the 7th, and I couldn't generate a hit to save my life. This happened once per series, and I lost each game. Fortunately, however, the next games were easy and I was able to defeat the Yankees for this achievement.