The checklist UI should appear on the main panel of the Jira issue.
If the checklist UI is not shown, click on the checklist icon at the top of the issue.
To ensure checklists are always visible, go to Jira Settings > Apps and select Global settings under the Issue Checklist heading. Use the toggle to enable Show checklist always (New Issue View).
To start creating a checklist simply type the first item in the text box at the top of the list. Hit enter or click on the checkmark when you’re done.