Subordinate or Nested Card or even Checklist within a Card

I have spent many hours the past few days trying to find a way to use the Kanban as a means to track post executive meeting follow up action items with my external customer. Some of these action items require multiple steps and I can’t find any resources that will show me how to either nest or create subordinate cards for a Kanban or even add a checklist to the interior of a card. Very frustrating. Can someone please help?