Not that hard and can be done without playing a match.
First off choose a top club (Real Madrid, Juventus, Bayern etc..)
Go to your team page and sell some of your players.
After some days (it took me 2 days) simulated. You will get a message that a club wants to buy your player.
When they ask for sell-on clause. Negotiate with the player and put it down to 1%. After that accept everything. And simulate to the next days.
After your player is sold the trophy will pop up