1. Have a clothing item with slots (as specified above) within your inventory but not equipped
2. Have all spaces within your inventory before the clothing item (and sometimes after, seems inconsistent) filled
3. Find an item to pick up that would be able to fit in the clothing item (a CR75 would fit in a UK Assault Vest, but a Cooking Tripod would not, etc.)
4. Pick up the item by
A. using the interact key in the world;
B. double-clicking the item in the inventory UI; or
C. clicking and dragging the item onto your character's image in the inventory UI
5. To view the item, drop the clothing item on the ground or equip it