Finish all missions on Hard difficulty. Use / to toggle the difficulty to Hard in the mission menu. You can change the difficulty to and from Hard whenever you want, you just need to be sure you have finished all missions on Hard at some point. One way to check is to enter a mission menu and change the difficulty to Hard, then back out to look at the list of missions. If each mission has at least 1 star, that means you beat it on Hard. You can do 5-star most missions and do all medals/badges/pins for each mission on Hard as well, but you can always clean those up on a different difficulty later.
Check out a guide for all medals, max ranks, and Hard tips (including DLC) here.