Once you have received a customization item, either by purchasing it on the marketplace or pulling it from a pack do the following:
From inside the ultimate team menu, go to your lineup by pressing RB until you get to my team, click edit lineup. Click on a player with A button and then apply customization item. Depending on what item it is they will give you 2-4 options depending on if it’s a skater or goalie. Press LT to cycle through options and once you have a customization item you press A and then apply it.