You don't need to “use” them. They simply DO. Do charms and talons stack? Yes, 1 of each.

Does charm and Talon stack?

Monster Hunter Rise Armorcharm and Powercharm stacking with Armortalon and Powertalon. You can stack them but this will require buying two of each with one in your inventory and one in the item box that you can craft into a talon before bringing that to your item box.

Do charms stack with armor?

Charm's effects indeed stack with each other. They can also stack with armor effects, increasing their effectiveness. The fact they can stack is important to create certain builds that can help you in your chosen playstyle.