As soon as you land, head towards the OKB Zero base. Don't slow down after reaching the guard post 01, as there are no enemies here. When you get closer to the base, you should stop running and start moving in a prone position, as well as be sure to make good use of the terrain to hide (tall grass, rocks, etc.). You should be extra careful about the first sniper, located on a metal balcony, next to a searchlight. He can locate Big Boss from a vast distance, even during the night time.
Don't attack the sniper, as it will alert the nearby enemies. Instead, start moving in a prone position towards the base. It's best to remain to the left of the main route. Unfortunately for you, the nearby area is filled with land mines, so you have to make sure to avoid them all. D-Dog will help you locate them. You should move at least several meters away from them, so that the risk of detonating any (and alerting the enemy to your position) is reduced to the minimum.
You can get to the base by using two different methods. Going through the main gate isn't the best idea, as the area there is well lit, and there's at least one Walker Gear patrolling the area. The best idea is to locate the wall, that can be climbed, which can be found to the left of the gate (as shown on the above screenshot). Using this method will allow you to get to the base without getting spotted.