To add cards to your playbook, simply click the "Add Card" button at the top right portion (right next to Duplicate and Edit buttons) of the playbook.
A list of cards that can you select from will appear. Search to quickly locate the specific card/cards you wish to add to the playbook.
Next, drag the card/cards to the destination column.
Once you're finished finding and dragging your chosen cards to the playbook, click Done and you're all set!