Go to the Smart Checklist menu, choose "Import from a Template".
You'll see the Templates dialog with the list of all Templates within the current Project.
Chose the one you'd like to add to your Jira issue and click "Import".
You can preview the template before importing it by clicking on its name and expanding the template's contents.
You will be asked for confirmation to append a Template to your current issue checklist.
A checklist will be imported into your issue.