In an online ranked game, press to open the GPS. Minor objectives are highlighted in yellow. All you have to do is go up to them and press to pick them up. This is best done in a 20 minute match. Some minor objectives can only be reached via step-jump. If you find that none are appearing on your GPS, go complete a main objective and they will respawn eventually. You might want to split with your partner so that one does the main objective while the other collects the minor ones. Both players on that team will get this achievement once it's done.