1. Go into Ultimate Team.
2. To create a co-op lobby, go to the main menu of FUT, go all the way to the left and click on the box showing two silhouettes/icons.
3. Invite your friend.
4. Click on squad battles, click any of the 4 squads (or the weekly featured squad), put the difficulty on Beginner, and play.
5. Rinse and repeat for 10 matches total. It's best to do this in one sitting so that the game counts all of them properly, previous FIFAs wouldn't always track it if you did it over several sessions (please correct me if it does work now).
You can also get the 10 Clean Sheets in Squad Battles achievement while doing this, so keep it on beginner difficulty, score a quick goal, and then just stand still with the ball while holding left bumper (LB) so that the game doesn't boot you for inactivity (the other player can also just hold LB even without the ball) to run down the clock.