Easily the most time consuming achievement in the game. You must collect 11 players, 1 badge and 2 kits from each team in the game, as well as 24 stadiums, 49 balls and 29 boots, giving you a grand total of 1614 cards! First of all I recommend downloading the Excel spreadsheet of the entire collection so you can mark things off as you go along:
http://www.seanbenton.org/wp-content/uploads/2009/03/champio... (thanks to Uranium Deposit and EOU Findub for their work with this)
From here you will want to repeatedly create and delete an Ultimate Team as it will give you about 15 bronze rated players each time, check them all off as you go along until you've got just about every one. The fun then starts as you have to buy Silver and Gold packs to collect all the cards, now that the servers have been shut down you can no longer trade online. There's not much else to it, people generally find that balls are the hardest ones to get so prioritise those. This achievement will take you literally hundreds of hours, so I hope you like the game!