When either manually adding an activity card or adding one to an engagement model in the administration you will notice there is nowhere to add a checklist to the card. You must create the card, fill in the appropriate information, save, and then reopen the card.
Once the activity card is reopened look for the checklist button on the left side next to “Add”
You can add as many checklist items as you want. Marking all of them off will not close the activity card, it exists to help keep you organized within one activity card.