All 100 trades must be done in the same career mode, so they do not carry over from save to save. When you are in career mode, scroll down and click on Transactions. When you click on it, you should have more options on your screen, scroll down and click on trades. You can trade with any team and you can trade for any player. It is easiest to make a new career or use a career that you are not trying to win anything and trade for a really bad player. Try to trade for a player that is bad overall and trade away one of your best players on your team. Once you offer the trade, go to the Career main menu and Advance to the next day. If there isn’t a popup showing that the trade was accepted, advance another day. Continue advancing until the trade is accepted or denied (it will most likely be accepted). Once there is a popup showing that the trade was accepted, click Finalize. Once you click this, advance one more day so the deal is officially finalized. Do this 100 times in one career to get the achievement.
It is easiest to do this while you are going for the 30 year career achievement because you will have a lot of time. Try to boost your Trade stat from the My Career menu. This will make it easier to do trades and you won’t have to trade good players for bad players every time.