In Metal Gear Solid 5: The Phantom Pain, how do I customize a weapon? I saw it in some pre-release gameplay for The Phantom Pain.
Customizing weapons is not very hard, you just need to know where to look.
First things first: You need to have completed a mission where you
Extract a Legendary Gunsmith
It's a Side Op, and is labeled as above. He'll become a part of your staff and you'll be able to customize weapons.
You're now done with the above, so how do you go about the actual "customization"?
First you need to be in the ACC (Aerial Command Center), you get there by just calling your chopper for a transport.
Next, you open your iDroid and go to the Mother Base tab and choose Customize.
And there you go, you should now have several options for customization.