You can use an external program, as described in other guides, to keybinding combat arts or you can use the built in-game feature as below.

If you open your combat art inventory tab and move your cursor over a CA and press F1, F2, F3, F4, F5, F6, the game will register these as key shortcuts.

To use them you have to select one slot for CA in the bottom right of your screen and press a shortcut key, the CA associated with the previous button will replace the current CA.

It is simple as that! In fact, I didn't know this for a long time...

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In the screenshot below, I replaced the fire ball with spiritual healing for example purposes: