Subitem Templates is included in the Monday.com's Public App Marketplace. You can locate our App on this page and then click 'Install' to add it to your account.
You can also install the App into your account from within the monday.com interface. Navigate to the Integrations option from any Project Board in your account:
From here you can simply search for 'Subitem Templates' to locate the App and install it into your account.
Make sure that the Subitems column is added to your Project board in Monday.com, or our App will not work. There is more information available here: https://support.monday.com/hc/en-us/articles/360017311219-Subitems-and-automations