This is a good code to use if you want to make a great team when you already started your season and saved.
From the main menu in "Season" go to "Front Office" then go to "Ownership Change" then make every team a user then make all the trades you want. Then once you have made the dream team that you have wanted, go and do the same thing over again except this time change all the users back to "CPU" and just keep the Team(s) you want.