Saving material to preparations while creating it
When creating a new material (test, presentation) or a new card, set the right topic in the plan and press the red + button. The test, presentation or card will automatically ataach to the selected topic. (Not valid for HW and project assignation. These materials have to be assigned to the topic explicitly).
You can check if the material has been attached to the plan when editing the material, in the "Assign" tab:
While creating the card: