THIS CHALLENGE IS NOT RETROACTIVE!
If you have completed any/all missions before this then you will have to complete them again.
However if you were in the middle of a mission before the challenge period and complete it during the challenge period, that one mission should count.
This is really simple compared to the previous "Like a Boss" challenges as you're both allowed to be spotted and kill enemies. It doesn't matter how well you do as long as you complete one of the mission objectives and get to the results screen.
You must do all the missions on both difficulties minus Déjà Vu which is the one where you must recreate scenes from previous Metal Gear Solid games.
If this is your first time playing all the side-ops will unlock when you complete the main Ground Zeroes mission except for "Jamais Vu" which requires you to find all the XOF patches during Ground Zeroes.
Multiple guides and videos can be found for the patches here: