Borderlands 3 and games like it are very much so a progress and you’ll find a way games. Even if it isn’t made apparent, it’s the type of thing you can’t actually use until much later, unless you preordered, did something related to the VIP program or purchased the Deluxe edition or better. Given the desire to utilize these mechanics, here is how you actually equip weapon trinkets and skin your weapons.
Equipping Weapon Trinkets
This is actually an option unlocked by default in Borderlands 3. All you need to do is click the box to the left of your weapon and it will bring up your available trinkets. If you didn’t purchase the season pass, sign up for the Vault Insider Program or get the Deluxe edition, you’ll have to wait until about an hour or two into the game before you gain the ability to buy them. The earliest and possibly only location is via Crazy Earl on the Sanctuary ship. He offers a few weapon skins and trinkets in exchange for a large amount of eridium.
Changing Weapon Skins
Similar to trinkets, weapon skins is more obtuse and out of the way than actually hard to find. Instead of having an outright button, a specific vender or NPC, you instead need to inspect a weapon. Upon inspecting, there will be an option at the bottom to change the skin. Similar to trinkets, you might not actually unlock any weapon skins or find a trinket until you progress to Sanctuary and purchase one from Crazy Early in Sanctuary.
It’s also important to note that certain items are more elaborate or noticeable than others. Since you can actually see your trinket, plus obviously the skin itself. the location will either be in your face or in the middle of nowhere. Certain weapon skins are also a bit more interesting than others. The Butt Stallion skin, which is shown above and called Butt Dazzle, actually has the sparkles moving on the gun itself. It’s a nice touch that does more than make it pink, at the cost of it being somewhat annoying if you focus on it.