Link resources to a card by dragging and dropping them from the top of the map. It is possible to link multiple resources to one order.
You can also do this from the card. Click on the card to open the 'back side'. In the shortcut menu, select it under resource. Use the dropdown to select the resource(s) you want to assign.
You can easily remove a resource linked to a map by clicking on the cross that appears on the avatar of the resource when you place your cursor on it.
When a card is linked to a resource, you will also see this in the resource overview.