When Big Boss will leave the helicopter you need to head to the R&D Platform marked by the game itself. You may either kill or fulton away the enemies you are going to face on your path however if you will fight in an open field it is easy to draw enemy attention and die. Even if the alarm will activate no reinforcements will appear. After getting to the platform use the stairs to get to the top of it.
You don't have to hurry, or actually - if you want, you may wait for enemies to regroup and calm down. After getting to the end of the stairs use your binoculars to identify the enemy commander. Mission 22 will end right after killing this person so you don't have to clear the entire area. You may use a good rifle as well as a grenade or a rocket launcher. The rest of the enemy forces will submit.
Warning - If you want to use any explosives pay attention to hit precisely the enemy commander or you may accidentally kill his hostages. It will not fail the mission although you will lose even a couple of hundreds heroic points.
In the end of this mission you will see a tutorial which will tell you about Security Team which specializes in defending the Mother Base from future attacks as well as the option of creating the first FOB (Forward Operating Base). Use from your iDroid the Base Facilities menu, then FOB1 and Purchase Waters. First transaction is free although after that you are going to have to spend the Motherboard Coins. You may read more about that in our second Metal Gear Solid V guide in the chapter focused on Mother Base.