You’ll gain access to User Generated Content after completing the story mission “Karma’s a Bitch", which, although it counts as a side mission, is needed to progress the story of the game. UGC missions show up on the map as a green "i" icon. For this trophy you need to select Pause > User-Generated Content > Mission Filters > Show: Newest. Once you've done this it may take a minute for the missions to start showing on your map but once they do they are all from the newest filter. So just proceed to any 5 of your choosing and complete them for this trophy.