On this page of our guide to Borderlands 3, we explain what Weapon Trinkets items are and what they are used for. You can find such items while playing the game, although there is also a possibility to buy them later during the main campaign.
Weapon Trinkets are cosmetic weapon ornaments. This means that they do not have any influence on the weapon's stats, but only allow you to change its appearance.
Acquired Weapon Trinkets ornaments end up in inventory. You have to select them from the inventory level, so that they will be permanently added to your collection of ornaments.
Decorations cannot be mounted on weapons until you advance to the main Sanctuary quest, i.e. to get on board the Sanctuary spacecraft for the first time, which in the game is used to travel between different planets.
After you start the Sanctuary quest, the first 2 weapon decoration slots will be unlocked in your inventory - we have marked them on the attached picture. Once you have selected a slot, you can place any decoration you like in it.
Decorations are very small and unfortunately it is difficult to see them even if you mount them on some weapon. The easiest way to see them is through the weapons menu.
You can also see the decoration during normal gameplay, depending on the weapon you are using. On some weapons the decorations can be noticed without a problem, but on others they can be barely visible.
Once you have advanced sufficiently in the Sanctuary quest, you will be able to visit Crazy Earl at the lowest level of the Sanctuary ship. Once you have initiated a conversation with Crazy Earl, check the Trinkets tab. For more decorations you have to pay with Eridium, not traditional cash.