Question about playing on a PS4 controller

[Niwashi]

When you are in your actions/traits screen, or in any other screen where you would want to key-bind something, press Square to bring up a key-bind interface. From here, just press the button combination you want to use for that ability. If you are in a screen like the inventory where this doesn't quite apply, ...

It applies the same for inventory items (or macros, or emotes, etc) as it does for Actions. Select an inventory item, press square, choose "Set to Hotbar", and press the button combination you want to use for that item.