Starting in our dashboard, we are looking to add a checklist to our task.

Click the Menu icon in the entry popover.

The Checklist interface will open up, start adding your items (press enter to confirm)

Should all be good to go, Click back on "New Task"

With checklist now open, proceed to check all the items

When all items are checked the progress bar is shown as full, and the task has been completed.

DayViewer - Adding A Checklist