Simplified Planner is installed in JIRA using Universal Plugin Manager (UPM). Following are the quick steps:
Log in as a JIRA System Administrator
Select Administration > Add-ons > Find new add-ons. The Universal Plugin Manager will be displayed
Enter "Simplified Planner' in the Search the Marketplace box and press Enter.
Click the Install button alongside the plugin name. A confirmation message and the plugin details will appear when the plugin is installed successfully.
See the JIRA documentation for more details on installing plugins. Once the plugin is installed, make sure the plugin is correctly configured.