Select "Customise" from the main menu.
Then select a character you wish to customise and choose any item from the many lists to buy for your character and make sure you press to equip it onto the character. Once you've equipped your snazzy new gear, try leaving the menu and you will get a little drop down asking to take a picture of your character. This is purely for the game to show you the custom character through a screenshot when selecting them for any game mode. Once you've taken the picture and accepted; the achievement will unlock.