You're going to be finding an obscene amount of guns in Borderlands 3, and to make them look the best that they can, you're going to want to outfit them with skins and trinkets. The skins are self-explanatory – they just change the way the gun looks and give the weapons some spiffy colors to match different designs – while the trinkets are neat little doodads that hang off your weapon, always in view in front of your character unless you're peering through a scope.
The problem is, Borderlands 3 never actually tells you how to equip your weapons with these fancy new looks. It's thankfully simple enough to do, and unlike the process for changing out character skins, you don't need to go to a Quick Change station to do it. This means that while you're picking up your unfathomable amounts of guns in your travels, you can customize them on the fly.
To start customizing weapon skins, simply head into the menu where all your guns are collected in your backpack and your current loadout is displayed. Highlight the gun that you want to customize and press the corresponding button to inspect the gun ("R3" on the Xbox One and PlayStation 4, "E" on the PC). From there, you'll have the prompt to press another button that brings up a list of available skins ("Square" on the PlayStation 4, "X" on the Xbox One, "X" on the PC) and you'll be able to pick a skin for your gun. Equipping one will show it right there in the menu so you can see if it's a good fit or not, and leaving the menu to show your weapon will give you a better idea of how it looks in-game.
Customizing trinkets is a different process. Return to the menu where you see your entire inventory laid out, but don't inspect the gun. Instead, you'll see small squares to the left of your equipped weapons. Highlight one of those boxes, select it, and you'll see your collection of trinkets show up. Equip one of those and return to your game and you'll see whatever the attachment is hanging off your weapon.
That's all there is to customizing weapons' skins and trinkets, but you're on your own when it comes to finding them. You can obtain them in your normal travels or purchase those and other cosmetics from Earl using Eridium. You may also occasionally get them through your mail which can be checked using the game's "Social" menu found in the paused screen.