After acquiring the necessary explosives, your path will lead you to the Metal Gear Hangar. This is a massive, multi-screen area, heavily fortified and patrolled by numerous guards, including some with heavy weaponry. The hangar itself is a maze of catwalks, platforms, and storage containers. Use your radar (if you have it) and Binoculars to plan your route carefully.
Metal Gear is located in the center of this hangar. It's a colossal bipedal tank. To destroy it, you must plant C4 explosives on its legs. There are four specific points on Metal Gear's legs where the C4 needs to be placed. Approach each leg, equip the C4, and press 'A' to plant it. Be aware that Metal Gear will activate and begin to patrol once you plant the first charge, making subsequent placements more challenging. Avoid its stomping attacks and machine gun fire.
After planting all four C4 charges, quickly retreat from the hangar. The countdown will begin, and Metal Gear will explode. However, your mission isn't over yet.