It is easiest to do this with a team with a high budget, such as the Boston Red Sox, the New York Yankees, or Chicago Cubs, but you can do this with any team you want. When you can first sign free agents in the offseason when you first start, sign as many good ones as you can to extremely long contracts. This way, you won’t have to worry about them becoming free agents again.
Once you have all of the other achievements and this is the last achievement you need, from the career menu, scroll all the way down and click on Settings, and then Sim Settings. Turn everything so that it says either Auto or Continue. This will prevent everything from stopping your simming and will make it go faster. To start simming, from the career menu, press Advance League and it will sim you until the first day of the next year. Once it stops simming, do it again. The simming will stop at the beginning of Spring Training, the Regular Season, and the Playoffs but once you pass these areas, you won’t have to press anything.